ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 



29 



There appears caudally on the blastoderm of reptiles a pit-like depression. 

 From this slight invagination a proliferation of cells forms a layer which spreads 

 beneath the ectoderm (cf. Fig. 21 A). The inner layer originating in this manner 

 is the entoderm, and the region of the pit where ectoderm and entoderm are con- 

 tinuous is the blastopore. 



In birds the caudal portion of the blastode rm is rolled or tucked under, the 

 inne r layer formed in this way constituting the entoderm. The marginal region 

 where ectoderm and entoderm meet bounds the blastopore, while the space be- 

 tween en toderm and yolk is the archenteron. 



Mammals. — As in cleavage, so also in gastrulation the mammalian ovum 

 exhibits a modified behavior indicative of an ancestral yolk-rich condition. The 

 ej itoderm apparently arises by a splitting off, or delam ina tinn , nf cells from the 

 u nder side of the inner cell mass (Figs. 16, 74 A and 75). In the blastoderm of the 

 rabbit, opossum, and mole, however, a minute pore has been observed at which 

 the ectoderm and entoderm are continuous. This opening is beheved by some to 

 represent a true blastopore where the ingrowth of entodermal cells has occurred. 



ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 

 Amphioxus and Amphibia. — The dorsa'- plate of entoderm, which forms the 

 roof of the archenteron in Amphioxus, gives rise to paired lateral diverticula or 

 coslomic pouches (Fig. 20). These separate both from the plate of cells in the mid- 



ect. 



mi s sow. 



cli .. 



I I. 



Dies. som. 



ect. 



Fig. 20. — Origin of the mesoderm in Amphioxus (after Hatschek). X about 425. n.g., Neural 

 groove; n.c, neural canal; ch., anlage of notochord; mes. som., mesodermal segment; ect., ectoderm; ent., 

 entoderm; al., cavity of gut; ae., coelom or body cavity. 



dorsal line (which forms the notochord), and from the entoderm of the gut, and 

 become the primary mesoderm. The mesodermal pouches grow ventral and their 

 cavities form the coelom' or ho&y cavity. Their outer walls, with the ectoderm, 



