DEVELOPMENT OF VERTEBRATES. 119 



pleted, though there is still a small mass of yolk filling up the 

 blastopore. The features of this involution are in the main 

 exaggerations of what was supposed to occur in the previous 

 animal. The asymmetry of the involution is so great that it is 

 completely one-sided and results, in the first instance, in a mere 

 slit ; and the whole process of enclosing the yolk by epiblast is 

 effected by the epiblast cells on the side of the egg opposite to 

 the involution. 



The true mesoblast and hypoblast are formed precisely as in 

 the previous case. The involuted cells become separated into 

 two layers, one forming the dorsal epithelium of the alimentary 

 canal, and a layer between this and the epiblast forming the 

 mesoblast. There is also a layer of mesoblast accompanying 

 the epiblast which encloses the yolk, which is derived from the 

 smaller yellow cells atj (C I). The edge of this mesoblast, ///, forms 

 a thickened ridge, a feature which persists in other vertebrates. 



It is a point of some importance for understanding the rela- 

 tion between the mode of formation of the alimentary canal in 

 the Frog and other vertebrates to notice that on the ventral 

 surface the cells which are to form the epithelium of the ali- 

 mentary canal become distinguished as such very much later 

 than do those to form its dorsal epithelium, and are derived not 

 from the involuted cells but from the primitive large yolk-cells. 

 It is indeed probable that only a very small portion of epi- 

 thelium of the ventral wall of the mid-gut is in the end derived 

 from these larger yolk-cells. The remainder of the yolk-cells 

 (C III, and C II, yk] form the yolk mass and do not become 

 directly formed into the tissues of the animal. 



In the last stage I have represented for the frog, C III, there 

 are several features to be noticed. 



The direct connection at their hind-ends between the cavities 

 of the neural and alimentary canals is the most important of 

 these. This is a result of the previous continuity of the epiblast 

 and hypoblast at the point x, and is a feature almost certainly 

 found in Amphioxus, but which I will speak of more fully in 

 my account of the Selachian's development. The opening of 

 the blastopore called, the anus of Rus.coni is now quite nar- 

 rowed, it does not become the anus of the adult. It may be 

 noticed that at the front end of the embryo the primitive dorsal 



