96 



TEXT-BOOK OF EMBRYOLOGY. 



compare with Fig. 46). Transverse sections of the disk are shown in Fig. 

 70, b, Si, Sz, SB, Si, 5). The mesoderm has increased and now comprises 

 an extensive layer between ectoderm and entoderm. From this stage 



emly. ea. 



FIG. 68. Median longitudinal section of an early embryo of Semnopithecus nasicus. Selenka, 

 from Quain's Anatomy, am, Amniotic cavity; con.stk., belly stalk; emby.ect., embryonic 

 ectoderm; ent, entoderm; mes, mesoderm; y.s., yolk sac. 



forward the course of development in the mammal follows the general line 

 of development in the bird. The mesoderm near the axial line becomes more 





B 



FIG. 69. Transverse sections through the embryonic disk of the lemur, Tarsius spectrum, after 

 the appearance of the primitive streak. Hubrecht, from Quain's Anatomy. A is a section 

 through the primitive node; B is a section through the primitive streak posterior to the node. 



massive and segmented transversely to form the mesodermal somites (Fig. 

 71). More laterally the mesoderm cleaves into two layers, the parietal and 

 visceral, the cleft itself representing the rudiment of the ccelom (Fig. 72). 



