32 ART. 11. — S. HATTA. 



It is a remarkable fact that a part of the macromeric cell- 

 mass, known as the 3^olk-plug in Amphibian ova, does not come 

 into view nt all. As before noted (p. 11), the yolk-plug is, in 

 fact, n part of the vegetative hemisphere, which is still ex- 

 posed after having already been surrounded by the furrow of the 

 sickle-groove brought about by infolding of the animal layer. In 

 the Petromyzon ovum, there is never an occasion for such a 

 temporary phenomenon to manifest itself, because the definitive 

 ventral lip of the blastopore is wanting in the younger stages and 

 is found only in the complete gastrula. 



A wedge-shaped structure composed of small cells and 

 found immediately inside the ventral blastoporic lip in the 

 gastrula. [Fig. 22, j).m.), represents the ventral part of the 

 peristomal mesoderm, which when traced anteriorly, is continuous 

 to the gastral part of the mesoderm already formed in this stage 

 on both sides of the " dorsal plate." It is proliferated out of 

 the micromeric epithelium forming the blastoporic lips. Earliest 

 traces of the proliferation is seen already in a little younger ova 

 {Fig. 23, 2^- ^«O- 



We have therefore before us archenteric walls, in which 

 the " dorsal plate " makes up the larger posterior section of the 

 roof of the cavity and in which the anterior part of the roof is 

 formed of the macromeric epithelium, while the lateral walls and 

 the floor are represented by indifferent macromeres. ■ The 

 " dorsal plate " gives rise, in its median part, to the gastral 

 mesoderm (Fig. 24, g. m.), and in the blastoporic lips, to the 

 peristomal mesoderm. The irregular macromeric epithelium 

 forming the anterior archenteric wall loses gradually its epithelial 

 structure and together with the lateral irregular epithelium, be- 

 comes indistinguishable from other macromeres; consequently 



