Embryology of the Siphonophora.
The school of Cuvier was lamentably deficient in embryologists; and it was only in the course of the first thirty years of the igth century that Prevost and Dumas in France, and, later on, Ddllinger, Pander, von Bar, Rathke, and Remak in Germany, founded modern embryology; and, at the same time, proved the utter incompatibility of the hypothesis of evolution as formulated by Bonnet and Haller with easily demonstrable facts.
As civil history may be divided into biography, which is the historyof individuals, and universal history, which is the history of the human race, so evolution falls naturally into two categories - the evolution of the individual (see Embryology) and the evolution of the sum of living beings.
If Meckel's proposition is so far qualified, that the comparison of adult with embryonic forms is restricted within the limits of one type of organization; and if it is further recollected, that the resemblance between the permanent lower form and the embryonic stage of a higher form is not special but general, it is in entire accordance with modern embryology; although there is no branch of biology which has grown so largely, and improved its methods so much since Meckel's time, as this.
In embryology the method finds its expression in the limitation of comparisons to the corresponding stages of low and high forms and the exclusion of the comparisons between the adult stages of low forms and the embryonic stages of higher forms.
Closely allied to the study of symmetry is the study of the direct effect of the circumambient media on embryonic young and adult stages of living beings (see Embryology: Physiology; Heredity; and Variation And Selection), and a still larger number of observers have added to our knowledge of these.
(See Embryology, Larval Forms. and Reproduction.) Finally, the conception of homogenesis does not exclude the differences between parent and offspring that continually occur, forming the material for the slow alteration of stocks in the course of evolution (see Variation And Selection).
The development of Aplysia from the egg presents many points of interest from the point of view of comparative embryology, but in relation to the morphology of the Opisthobranchia it is sufficient to point to the occurrence of a trochosphere and a veliger stage (fig.
Embryology The Egg.
Kowalevsky or a bug is invaginated into the yolk at the head end, the portion of (1871 and 1887) on the embryology of the water-beetle Hydrophilus the blastoderm necessarily pushed in with it forming the amnion.
Carriere's researches (1897) on the make its appearance simultaneously throughout the whole length of embryology of the mason bee (Chalicodoma) agree entirely with the the plate; the anterior parts are segmented before the posterior.
Escherich (1901), after a new of these twenty-one divisions are so different from the others that research on the embryology of the muscid Diptera, claims that the they can scarcely be considered true segments.
That some of the later work on insect embryology has justified the It is now ascertained that the procephalic lobes consist of three growing scepticism in the universal applicability of the " germ-layer divisions, so that the head must certainly be formed from at least theory."
Heider, Handbook of the Comparative Embryology of Invertebrates (trans.
During all this time little had been done in studying the internal structure of birds; 3 but the foundations of the science of embryology had been laid by the investigations into the development of the chick by the great Harvey.
The embryology of insects is entirely a study of the last century.
Most of the recent work on the embryology of insects has been done in Germany or the United States, and among numerous students V.
Embryology shows that originally these different parts are separately started, and only ultimately become united into one.
Development.-The embryology of the Nemertines offers some very remarkable peculiarities.
Soc. xli., 1902), little real advance has been made in our knowledge of the embryology of the Brachiopoda within recentears.
Xix, ninth edition, PP 44 0 -44 1) has not met with acceptance, and until we have a fuller account of the embryology of some one form, preferably an Inarticulate, it is wiser to regard the group as a very isolated one.
An important fact in its favour was discovered by Laurie (17), who investigated the embryology of two species of Scorpio under Lankester's direction.
For the loan of several figures from the translations published by them of the admirable treatise on Embryology by Professors Korschelt and Heider; also to the publishers of the treatise on Palaeontology by Professor Zittel, Herr Oldenbourg and The Macmillan Co., New York, for several cuts of extinct forms.
For information as to the embryology of scorpions, the reader is referred to the works named in the bibliography below.
Laurie, M., " The Embryology of a Scorpion," Quart.
Iii., 1899 Morgan, " Embryology and Phylogeny of the Pycnogonids," Biol.
Embryology: - Balfour, " Development of the Araneina," Q.
Xx., 1880; Kingsley, " The Embryology of Limulus," Journ.
Thus Bionomics is treated in such articles as Evolution, Heredity, Variation, Mendelism, Reproduction, Sex, &C.; Zoo-dynamics under Medicine, Surgery, Physiology, Anatomy, Embryology, and allied articles; Plasmology under Cytology, Protoplasm, &C.; and Philosophical Zoology under numerous headings, Evolution, Biology, &C. See also Zoological Distribution, Palaeontology, Ocranography, Microtomy, &C.
Gradually since the time of Hunter and Cuvier anatomical study has associated itself with the more superficial morphography until to-day no one considers a study of animal form of any value which does not include internal structure, histology and embryology in its scope.
Von Baer, however, has another place in the history of zoology, being the first and most striking figure in the introduction of embryology into the consideration of the relations of animals to one another.
His true greatness can only be estimated by a consideration of the fact that he was a great teacher not only of human and comparative anatomy and zoology but also of physiology, and that nearly all the most distinguished German zoologists and physiologists of the period 1850 to 1870 were his pupils and acknowledged his leadership. The most striking feature about Johann Miller's work, apart from the comprehensiveness of his point of view, in which he added to the anatomical and morphological ideas of Cuvier a consideration of physiology, embryology and microscopic structure, was the extraordinary accuracy, facility and completeness of his recorded observations.
In the edition of the Natiirliche Schopfungsgeschichte published in 1868 he made a great advance in his genealogical classification, since he now introduced the results of the extraordinary activity in the study of embryology which followed on the publication of the Origin of Species.
But it was only after Darwin that the cell-theory of Schwann was extended to the embryology of the animal kingdom generally, and that the knowledge of the development of an animal became a knowledge of the way in which the millions of cells of which its body is composed take their origin by fission from a smaller number of cells, and these at last from the single egg-cell.
Kolliker (Development of Cephalopods, 1844), Remak (Development of the Frog, 1850), and others had laid the foundations of this knowledge in isolated examples; but it was Kovalevsky, by his accounts of the development of Ascidians and of Amphioxus (1866), who really made zoologists see that a strict and complete cellular embryology of animals was as necessary and feasible a factor in the comprehension of their relationships as at the beginning of the century the coarse anatomy had been shown to be by Cuvier.
The term " grade " was introduced by Ray Lankester (" Notes on Embryology and Classification," in Quart.
On the ether hand, a survey of the facts of cellular embryology which were accumulated in regard to a variety of classes within a few years of Kovalevsky's work led to a generalization, independently arrived at by Haeckel and Lankester, to the effect that a lower grade of animals may be distinguished, the Protozoa or Plastidozoa, which consist either of single cells or colonies of equiformal cells, and a higher grade, the Metazoa or Enterozoa, in which the egg-cell by " cell division " gives rise to two layers of cells, the endoderm and the ectoderm, surrounding a primitive digestive chamber, the archenteron.
But practical necessity has given rise to the existence of many other divisions; see CYTOLOGY, for the structure of cells; EMBRYOLOGY, for the development of individual organisms; HEREDITY and REPRODUCTION, for the relations between parents and offspring.
The study of comparative pathology, yet in an inchoate stage, and of embryology, illuminated and enlarged biological conceptions, both normal and abnormal; and the ens reale subsistens in corpore disappeared for ever - at any rate from physiology and medicine.
If the Phylactolaemata were evolved from the type of structure represented by Phoronis or the Pterobranchia, the Gymnolaen ata should be a further modification of this type, and the comparative study of the embryology of the.two orders would appear to be meaningless.
The development of the compound microscope rendered possible the accurate study of their life-histories; and the publication in 1851 of the results of Wilhelm Hofmeister's researches on the comparative embryology of the higher Cryptogamia shed a flood of light on their relationships to each other and to the higher plants, and supplied the basis for the distinction of the great groups Thallophyta, Bryophyta, Pteridophyta and Phanerogamae, the last named including Gymnospermae and Angiospermae.
These important discoveries mark a new epoch in embryology, and may be said to be the foundation of the views now entertained, which were materially aided by the subsequent elucidation of the process of cytogenesis, or cell-development, by Schleiden, Schwann, Mohl and others.
The mutual relations of palaeontology and embryology and comparative anatomy as means of determining the ancestry of animals are most interesting.
The student must therefore resort to what may be called a tripod of evidence, derived from the available facts of embryology, comparative anatomy and palaeontology.
Such sudden appearances may be demonstrated possibly in zoology and embryology but never can be demonstrated by palaeontology, because of the incompleteness of the geological record.
Certain modifications in the disposition of these classes are naturally enough rendered necessary by the vast accumulation of knowledge as to the anatomy and embryology of the forms comprised in them.
The evidence of embryology is decidedly against the view that the eye-stalks are limbs.
They are of interest, however, rather from the point of view of general embryology than from that of the special student of the Crustacea, and cannot be fully dealt with here.
On the data afforded by comparative anatomy and embryology in attempting to reconstruct the probable phylogeny of the class.
The embryology of Myzostoma has been C A, Ventral view of Myzostoma.