A Treatise on comparative embryology, Volume 2

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Macmillan and Company, 1881
 

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Page 312 - ... to be situated at the anterior end of the body. If this view is correct, the question presents itself as to how far the posterior part of the nervous system of the Bilateralia can be regarded as derived from the primitive radiate ring. " A circumoral nerve-ring, if longitudinally extended, might give rise to a pair of nerve-cords united in front and behind, — exactly such a nervous system, in fact, as is present in many Nemertines (theEnoplaand Pelagonemertes), in Peripatus and in primitive...
Page 568 - ... median division of the body-cavity by longitudinal septa of transverse muscles. Each fully developed organ consists of three parts : (1) A dilated vesicle opening externally at the base of a foot. (2) A coiled glandular tube connected with this and subdivided again into several minor divisions. (3) A short terminal portion opening at one extremity into the coiled tube (2) and at the other, as I believe, into the bodycavity.
Page 286 - ... are derived from the epiblast ; but the limits of both these sections are so variable, sometimes even in closely allied forms, that it is difficult to avoid the conclusion that there is a borderland between the epiblast and hypoblast, which appears by its development to belong in some forms to the epiblast and in other forms to the hypoblast. If this is not the case it is necessary to admit that there are instances in which a very large portion of the alimentary canal is formed of epihlast.
Page 283 - ... that either of them reproduces the mode of passage between the Protozoa and the ancestral two-layered Metazoa. These conclusions do not, however, throw any doubt upon the fact that the gastrula, however evolved, was a primitive form of the Metazoa ; since this conclusion is founded upon the actual existence of adult gastrula forms independently of their occurrence in development.
Page 92 - ... to the blastoderm of meroblastic ova; while the vegetative pole, which subsequently forms a large yolk-sack, is divided by a few vertical furrows, four of which nearly meet at the pole opposite the blastoderm. The majority of the vertical furrows extend only a short way from the edge of the small spheres, and are partially intercepted by imperfect equatorial furrows.
Page 294 - Amphioxus ; but this process approaches in many forms of Vertebrata to the ingrowth of the mesoblast from the lips of the blastopore. It is, therefore, highly probable that the paired ingrowths of the mesoblast from the lips of the blastopore may have been in the first instance derived from a pair of archenteric diverticula.
Page 286 - Hydra, especially the sensory and protective layer, while the hypoblast was the secretory and assimilating layer; both layers giving rise to muscular elements. It must not, however, be supposed that in the early diploblastic ancestors there was a complete differentiation of function, but there is reason to think that both the primary layers retained an indefinite capacity for developing into any form of tissue1. The fact of the triploblastic condition being later than the diploblastic proves in a...
Page 151 - No attempt has been made in the figure to indicate the position of the dorsal wall of the throat, which cannot be easily made out in the living embryo; ch.
Page 549 - In Elasmobranch Fishes we thus have (1) a series of paired bodies derived from the sympathetic ganglia, and ("2) an unpaired body of mesoblastic origin. In the Amniota these two bodies unite to form the compound suprarenal bodies, the two constituents of which remain, however, distinct in their development. The mesoblastic constituent appears to form the cortical part of the adult suprarenal body, and the nervous constituent the medullary part.
Page 344 - It seems to me probable, though further investigations on the point are still required, that the dorsal fissure is a direct result of the atrophy of the dorsal part of the central canal of the spinal cord. The walls of the canal coalesce dorsally, and the coalescence gradually extends...

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