Here too they disappear during embryonic life.
In Ieptolinae the embryonic development culminates in a polyp, which is usually formed by fixation of a planula (parenchymula), rarely by fixation of an actinula.
The observation of the existence of an analogy between the series of gradations presented by the species which compose any great group of animals or plants, and the series of embryonic conditions of the highest members of that group.
The general proposition as to a parallelism between individual and ancestral development is no doubt indisputable, but extended knowledge of the very different ontogenetic histories of closely allied forms has led us to a much fuller conception of the mode in which stages in embryonic and larval history have been modified in relation to their surroundings, and to a consequent reluctance to attach detailed importance to the embryological argument for evolution.
(I) Amphicoelous; each end of the centrum is concave; this, the lowest condition, is embryonic, but was retained in Archaeopteryx and in the thoracic vertebrae of Ichthyornis.
The middle classes, which were making other countries rich and powerful, existed only in an embryonic condition.
As in other Molluscan groups, we find a wide variation in the early process of the formation of the first embryonic cells, and their arrangement as a diblastula, dependent on the greater or less amount of food-yolk which is present in the egg-cell when it commences its embryonic changes.
In other Pectinibranchia (and such variations are representative for all Mollusca, and not characteristic only of Pectinibranchia) we find that there is a very unequal division of the egg-cell at the commencement of embryonic development, as in Nassa.
These two endoderm-rudiments embryonic membrane formed by delamination from the blastoderm, ultimately grow together and give rise to the epithelium of the midwhile in a few insects, including the wingless spring-tails, the emgut.
Schwangart, who has studied (1904) the embryonic insects, twenty-one of these divisions - not, however, all similar - development of Lepidoptera.
Segment; while the fourth, fifth, and sixth are respectively the Embryonic Membranes.
- A remarkable feature in the embryonic segments of the mandibles and of the first and second maxillae.
Though the T hx, thoracic segments bear the wings, no trace of these appendages exists till the close of the embryonic life, 8 `' nor even, in many cases, till much later.
The pseudopods that exist on the abdomen of numerous caterpillars may possibly arise from the embryonic pseudopods, but this also is far from being established.
The ejaculatory duct which opens on the ninth abdominal sternum in the adult male arises in the tenth abdominal embryonic segment and subsequently moves forward.
During post-embryonic development - no insect being hatched with the smallest external rudiments of those organs - and on the necessity for successive castings or " moults " (ecdyses) of the cuticle.
Pratt has traced them in the sheep-tick (Melophagus) to an early stage of the embryonic life.
As to the nature of histogenesis, nothing more can be said than that it appears to be a phenomenon similar to embryonic growth, though limited to certain spots.
Insect metamorphosis may be briefly described as phenomena of development characterized by abrupt changes of appearance and of structure, occurring during the period subsequent to embryonic development and antecedent to the reproductive state.
The post-embryonic growth of Hexapods with or without metamorphosis is accompanied in most cases by the acquisition of wings.
For in the post-embryonic development of the ancestors of the Endopterygota we may imagine two or three instars with wing-rudiments to have existed, the last represented by the sub-imago of the may-flies.
It is at times sacculated, but its chief interest is that, as Lebedinsky 1 has shown, the tip of the caecum in embryonic life opens to the exterior as the blastopore.
- lv.) found vestiges of a keel in a young rhea, and apteria in the embryonic ostrich, and she concluded that they were descendants of birds which originally possessed the power of flight.
The young Brachiopod in all its species is protected by an embryonic shell called the " protegulum," which sometimes persists in the umbones of the adult shells but is more usually worn off.
In living Arachnids, excepting the Pantopoda, it is either fused (with loss of its appendages) with the prosoma (Limulus, 1 Scorpio), after embryonic appearance, or is 1 Pocock suggests that the area marked vii.
This wasting may be general or local - continuously from the embryonic period there is this natural process of displacement and decay of tissues going on in the growing organism.
Cohnheim's hypothesis of " embryonic residues " provides that early in the development of the embryo some of the cells, or groups of cells, are separated from their organic continuity during the various foldings that take place in the actively growing embryo.
The greater the degree of anaplasia the more the tumour cells conform in character and appearance to the embryonic type of cell and the more malignant is the new growth.
The thicker portion develops a terminal muscular rostellum and two or four suckers, the thinner end (" tail ") is vesicular, more or less elongated, and contains the six embryonic hooks.
Of Macedon had begun his career of conquest, and had shattered an embryonic alliance between the league and certain princes of Thrace (Cetriporis), Paeonia (Lyppeius) and Illyria (Grabus).
Trained in a school where the principles of responsible government were still in an embryonic state, where the adroit management of coalitions and cabals was essential to the life of a political party, and where plots and counterplots were looked upon as a regular part of the political game, he acquired a dexterity and skill in managing men that finally gave him an almost autocratic power among his political followers.
The careful study of the development of one Acoelous form and of certain Rhabdocoels has strengthened this hypothesis by showing that no definite enteron or gut is at first laid down, but that certain embryonic syncytial tracts become digestive tracts, others excretory, others again muscular.
Unlike the cells of Protozoa, these embryonic cells of the Metazoa do not remain each like its neighbour and capable of independent life, but proceed to arrange themselves into two layers, taking the form of a sac. The cavity of the two-cell-layered sac or diblastula thus formed is the primitive gut or arch-enteron.
By division of the egg-cell a mulberry-mass of embryonic-cells is formed (morula), which dilates, forming a one-celllayered sac (blastula).
- " Veliger " embryonic form of Mollusca.
About the fifth week of human embryonic life the tunica albuginea appears in the male, from which septa grow to divide the testis into lobules, while the epithelial cords form the seminiferous tubes, though these do not gain a lumen until just before puberty.
Successive observers in Italy, notably Fracastoro (1483-1553), Fabio Colonna (1567-1640 or 1650) and Nicolaus Steno (1638 - c. 1687), a Danish anatomist, professor in Padua, advanced the still embryonic science and set forth the principle of comparison of fossil with living forms. Near the end of the 17th century Martin Lister (1638-1712), examining the Mesozoic shell types of England, recognized the great similarity as well as the differences between these and modern species, and insisted on the need of close comparison of fossil and living shells, yet he clung to the old view that fossils were sports of nature.
The evolution consists first in progressive increase in size; second, in the acceleration of the median digit and retardation of the lateral digits, the latter becoming more and more elevated from the ground until finally in Equus (6) they are the lateral splints, which in the embryonic condition have vestigial cartilages attached representing the last traces of the lateral phalanges.
This very characteristic larva is termed a planula, but though very uniform externally, the planulae of different species, or of the same species at different periods, do not always represent the same stage of embryonic development internally.
It is possible that these were embryonic " batteries traditores " to flank the intervals.
In all the embryonic or permanent opening is on the coxa of the fifth pair of prosomatic limbs.
I is the embryonic condi tion.