42 



SEGMENTATION OF THE FERTILIZED OVUM 



ing from the dorsal surface of the germinal area into the space beneath the ento- 

 derm, the archenteron. No gastrulation stage for the human embryo has yet 

 been observed but the primitive streak may be recognized in later stages (Fig. 

 73 A). There is also evidence of an opening, the notochordal or neurenteric canal, 

 leading from the exterior into the cavity of the primitive gut (archenteron). 



According to the view of Keibel and Hubrecht, the invagination of cells to form the noto- 

 chordal plate in reptiles, birds and mammals is a secondary process not to be compared with 

 formation of the entoderm by gastrulation, as in Amphioxus. The notochordal plate is not 

 entodermal but ectodermal, and the primitive streak cannot be compared in its entirety to the 

 blastopore of Amphioxus. 



THE ORIGIN OF THE MIDDLE GERM LAYER (Mesoderm), NOTOCHORD, AND 



NEURAL TUBE 



Amphioxus. — The dorsal plate of entoderm, which forms the roof of the 



archenteron, gives rise to paired lateral diverticula or ccelomic pouches (Fig. 25). 



These separate both from the plate of cells in the mid-dorsal line (which form the 



notochord) , and from the entoderm of the gut, and become the primary mesoderm. 



mcs. 



Fig. 25. — Origin of the mesoderm in Amphioxus (after Hatschek). 11. g., neural groove; n.c, neural 

 canal; ch., anlage of notochord; mes. sow., mesodermal segment; ect., ectoderm; ent., entoderm; a/., 

 cavity of gut; roc, coelom or body cavity. 



The mesodermal pouches grow ventrad and their cavities form the coelom or 

 body cavity. Their outer walls, with the ectoderm, form the body wall or 

 somatopleurc; their inner walls with the gut entoderm, form the intestinal wall 

 (s planchnopleure) . In the meantime, a dorsal plate of cells cut off from the ec- 

 toderm has formed the neural tube (anlage of central nervous system), and the 

 notochordal plate has become a cord or cylinder of cells extending the length of 

 the embryo (axial skeleton). In this simple fashion the ground plan of the 



