VERTEBRATES. 



Fig. 3. — Transverse section of embryo 

 newt ; e, epiblast ; hy, hypoblast ; me, 

 mesoblast ; mg, medullary groove. 



organs of the body may be ti-aced, viz : epiblast, mesoblast, and hypoblast. Of these, 

 the former is constituted by the outer small cells, which now encase the whole embryo ; 

 the latter lines the intestine and draws upon the larger yolk-laden cells, as additions 

 to the layer are required, while these two layers are separated by the mesoblast, which, 

 in its development, is closely connected with the invagination of the intestine. 



Although this orifice of invagination, or blastopore, is very much restricted in 

 extent in the frogs, it would seem to have been much longer in earlier forms, for it is 

 still represented in some amphibians as a ' primitive groove,' which extends along the 

 dorsal surface of the egg, — that afterwards occupied by the central nervous system. 

 Again, in some tailed amphibians the intestine retains, j)ermanently, a communication 

 with the outside through the blastopore, which is converted into the anus, and it is 

 possible that this communication may have been double in earlier forms ; a mouth per- 

 sisting at the front end of the elongated blastopore, 

 while the intervening part of the slit became closed. 

 Neither slit nor groove are to be detected in the frog, 

 but along that surface which is thus occupied in other 

 forms, the epiblast becomes altered into a series of cells, 

 which, at first, form a flat plate on the level of the other 

 epiblastic cells, but afterwards come to lie in a groove 

 below that level. These altered epiblastic cells are des- 

 tined to form the central nervous system. The groove 

 is known as the medullary groove ; in the middle of 

 its extent it soon becomes converted into a tube, so that the resultant nervous tube 

 or canal then communicates with the outside merely by an anterior and posterior 

 neuropore. During the closure of the medullary groove, which is illustrated by 

 a transverse section of an embryo newt in Fig. 3, the epiblast, which was at the sides 

 of the groove, comes to lie over it, and a seam is thus formed along the middle line 

 above, all traces of which are, however, very soon obliterated. 



It is very probable, as has been suggested by Sedgwick and Van Wijhe, that the 

 stage of development we are considering now, 

 where the neural tube is open in front and 

 behind, really answers to an ancestral verte- 

 brate form, where both intestine and neural 

 tube were traversed by streams of water 

 sucked in by the ciliated linings of these canals. 

 The anterior neuropore is only very transi- 

 torily open, however (except in Amphioxus, 

 where, even in the adult, such a stream enters 

 the neural tube), but the posterior neuropore 

 retains for some time after that an intimate 

 relation with the blastopore (Fig. 4), or when 

 that orifice closes, as it does in the case of the 

 toad here described, with the neighboring part of the intestine, so that a canal of com- 

 munication between the neural and intestinal tubes (neurenteric canal) exists for some 

 time. That rudiments of this canal are met with also in the embryos of birds and 

 mammals is strong evidence of the truth of this hypothesis. 



So far the intestine of the embryo toad only communicates with the exterior by 

 the blastopore, but, when that closes, a new aperture, the anus, is formed further for- 



FlG. 4. — Diagrammatic longitudinal section of em- 

 bryo toad ; al, alimentary tract; «c, neural canal; 

 m, mesoblast; y, hypoblast ; n, neurenteric canal. 



