204 OUTLINES OF CHORDATE DEVELOPMENT 



median dorsal side of the endodermal wall of the intestine. 

 The cells composing this ridge resemble closely the other cells 

 of this part of the enteric wall, and are to be distinguished 

 chiefly by their behavior (Fig. 76, A .) The mesentery is formed 

 shortly after this stage, and when the mesodermal folds push 



a 

 sc 



cv 



sc 



FIG. 76. Sections showing the origin of the sex-cells (germ cells) in R. 

 sylvatica. After Allen. A. B. Sections of a 7.5 mm. larva showing (A) sex-cell 

 ridge of endoderm and (B) its separation as the sex-cell cord. C. Part of a section 

 of an 8.3 mm. larva showing the beginning of the migration of the sex-cells. 

 a, Dorsal aorta; ch, notochord; cv, posterior cardinal vein; e, endoderm cells; 

 g, gut cavity; I, lateral plate of mesoderm; m, mesentery; my, myotome; n, nerve 

 cord; sc, sex-cell cord; sch, subchordal rod (hypochorda) ; sr, sex-cell ridge; 

 W, Wolffian duct. 



in toward the mid-line, above the gut, this ridge of primitive 

 germ cells seems to separate from the gut and to move dorsally, 

 so that when the mesentery is formed they are found in its 

 base, near the body wall (Fig. 76, B, C). Here they form a 



