After the gastrula stage, which is found as a developmental stage in all Enterozoa, the embryo of the Hydrozoa proceeds to develop characters which are peculiar to the Coelen- a terata only.
At the present moment it is impossible to draw up any definition, based on broad anatomical or developmental characters, by which any one of Cuvier's great groups shall be separated from all the rest.
Such a system is called a tissuesystem, the word tissue being employed for any collection of cells with common structural, developmental, or functional characters to which it may be conveniently applied.
This surface layer in the typically subaerial shoot of the sporophyte in Pteridophytes and Phanerogams is known as the epidermis, though the name is restricted by some writers, on account of developmental differences, to the surface layer of the shoot of Angiosperms, and by others extended to the surface layer of the whole plant in both these groups.
The alternative, which is here accepted, is that differentiation is essentially the expression of a developmental tendency inherent in the protoplasm of plants.
Nature possesses three great educational or developmental schools - terrestrial, aquatic and aerial life.
Loisy's developmental defence of Catholicism; Professor Harnack's review of L'Evangile et l'Eglise in the Theol.
Similarly Karl Hoffmann of Wiirzburg wasted his appreciations of the newer schools of developmental biology in fanciful notions of human diseases as reversions to normal stages of lower animals; scrofula being for him a reversion to the insect, rickets to the mollusc, epilepsy to the oscillaria, and so forth.
The life-history of Schistostomum haematobium is still unknown, but the difficulty in obtaining developmental stages in any of the numerous intermediate hosts that have been tried suggests that the ciliated larvae may develop directly in man and either gain access to him by the use of impure water for drinking or may perforate his skin when bathing.
Stuhlmann's work (54a) on developmental phases of T.
It is impracticable here to consider fully all the various developmental phases and modifications of the life-cycle described as occurring in the above parasites.
In 1866 the Russian embryologist Kowalewsky startled the zoological world with a minute account of the developmental changes of Ascidia, one of the Tunicata, 5 and it became evident that the affinities of that class were with the Vertebrata, whilst their structural agreements with Mollusca were only superficial.
There is nothing in their mode of life which will afford an explanation of the asymmetry which is a developmental phenomenon.
The widening sphere of state activity, so marked a characteristic of modern civilization, involves outlay for what may be best described as " developmental " services.
Though the corallum appears to live within the zooid, it is morphologically external to it, as is best shown by its developmental history.
It might be possible to prove the origin of all classes from Pelmatozoa, without thereby explaining the origin of such fundamental features as radial symmetry, the developmental metamorphosis, and the torsion that affects both gut and body-cavities during that process; but the acceptance of a Dipleurula as the common ancestor necessitates an explanation of these features.
There is much to be said in favour of the view entertained by some entomologists that the structural and developmental characteristics of may-flies are sufficiently peculiar to warrant the formation for them of a special order of insects, for which the names Agnatha, Plectoptera and Ephemeroptera have been proposed.
Lankester, " Contributions to the Developmental History of the Mollusca," Phil.
The sexual cells are borne on the mesenteries in positions irrespective of obvious developmental radii.
Putting aside the last-named, for a detailed account of which see Hydromedusae, we can best deal with the peculiarities of the polyp and medusa from a developmental point of view.
Some authors looking upon these as parthenogenetic ova regard the developmental cycle as one composed of an alternation of parthenogenetic and of sexual generations.
His most important memoirs, besides that just mentioned, are those on the anatomy and classification of Fishes, on the Caecilians and on the developmental history of the Echinoderms.