6 INTRODUCTORY EMBRYOLOGY. 



The outer of the two layers of the gastrula is called the 

 ectoderm, the inner the entoderm. 1 The cavity bounded by the 



FIG. 5. Diagram of a gastrula and of the later closure of the blastopore., 

 a, archenteron ; b, blastopore ; ec, ectoderm ; en, entoderm ; S, segmentation cavity. 



entoderm is the archenteron (stomach), and the opening where 

 ectoderm and entoderm meet, and where the archenteron com- 

 municates with the external world, is the blastopore. Usually 





1 i<;. 6. Sat^lital section of early embryo (late gastrula) of Ambtystoma. b, 

 blastopore; t t In-ginning of infolding of brain; <-, entoderm; HI, thickening of 

 ectoderm for mouth, hypophysis, and nose ; tns, mesoderm ; y, yolk-mass. 



the blastopore is an elongate slit, its major diameter coinciding 

 with the longitudinal axis of the future animal. Soon after 

 imagination, the blastopore begins to close, the opposite lips 



1 In ^lish works these two layers are called respectively epiblast and 



hypoblast, wliil.- th< inrsodnn,. to l)e mentioned later, is called mesoblast. There is no 

 :t using these terms. 



