344 



GENERAL EMBRYOLOGY 



pm 



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 enterocoels; 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. 



