6 INTRODUCTORY EMBRYOLOGY. 



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

 ectoderm, the inner the entoderm.^ 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 



Fig. 6. Sagtital section of early embryo (late gastrula) of Amblystoma. b, 

 blastopore; c, beginning of infolding of brain; t, entoderm; in, thickening of 

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



the blastopore is an elongate slit, its major diameter coinciding 

 with the longitudinal axis of the future anirnal. Soon after 

 invagination, the blastopore begins to close, the opposite lips 



I In many English works these two layers are called respectively epiblast and 

 hypoblast, while the mesoderm, to be mentioned later, is called mesoblast. There is no 

 longer necessity for using these terms. 



