124 EARLY STAGES IN THE 



tail of the embryo) as well as everywhere else. It thus comes 

 about that the final point where the various parts of the blasto- 

 derm meet and completely enclose the yolk mass does not cor- 

 respond with the anus of Rusconi of the Frog, but is at some 

 little distance from the hind-end of the embryo. In other 

 words, the position of the blastopore in the Selachian is not 

 the same as in the Frog. 



Another point deserving attention is the formation of the 

 ventral wall of the alimentary canal. This takes place in two 

 ways partly by a folding-in at the sides and end, and partly 

 from cells formed around the nuclei () in the yolk. From 

 these a large portion of the ventral wall of the mid-gut is 

 formed. 



The folding-in of the sheet of hypoblast to assist in the 

 closing-in of the ventral wall of the alimentary canal is a conse- 

 quence of the flattened form of the original alimentary slit which 

 is far too wide to form the cavity of the final canal. In the Bird 

 whose development must next be considered this folding-in is a 

 still more prominent feature in the formation of the alimentary 

 canal. As in the last case, the alimentary canal is widely open 

 in the middle to the yolk at the time when its two ends are 

 closed below and shut off from it ; still later this opening be- 

 comes very narrow and forms the duct of the so-called umbilical 

 cord which places the yolk-sac in communication with the ali- 

 mentary canal. As the young animal becomes larger the yolk- 

 sac ceases to communicate directly with the alimentary canal, 

 and is carried about by it for some time as an appendage and 

 only at a later period shrivels up. 



The mesoblast is formed in a somewhat different way in the 

 Sharks than in other vertebrates. It becomes split off from the 

 hypoblast, not in the form of a single sheet as in other verte- 

 brates, but as two lateral sheets, one on each side of the middle 

 line and separated from one another by a considerable interval ; 

 whilst the notochord is derived not as in other vertebrates from 

 the mesoblast, but from the hypoblast (vide F. M. Balfour, " De- 

 velopment of Selachians 1 ," Journal of Microscopical Science, Oct., 

 1874). 



1 Paper No. V, p. 82 el set/, in this edition. 



