344 



GENERAL EMBRYOLOGY 



pm 



en 



ch 



FIG. 158. Semi-diagrammatic drawings of sections through Elasmobranch 

 embryos. After Greil, after Riickert and Ziegler. A-D. Sagittal sections; 

 E, F, transverse sections. A. Diagram of section to show relations of blasto- 

 derm to animal pole of blastula with less yolk. B. Commencement of gastrula- 

 tion by invagination, epiboly, and involution. C, D. Continuation of all three 

 processes of gastrulation. E. Transverse section showing relation of endoderm 

 to yolk, mesoderm, and the germ ring. F . Transverse section farther forward, 

 through stage resembling D. Anterior margin of blastoderm toward the left in 

 A-D. a, archenteron; c, vestiges of enteroccels; ch, rudiment of notochord; 

 ec, ectoderm; en, endoderm; gm, gastral (axial) mesoderm; gr, germ ring; m, 

 mesoderm; n, neural groove, bordered laterally by neural folds; pm, peristomial 

 mesoderm; s, blastoccel or sub-germinal cavity; y, yolk. 



