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; <, 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 



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

 blastopore; c, beginning of infolding of brain; ^, entoderm; ;//, thickening of 

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



the blastopore is an elongate slit, its major diameter coinciding 

 with the longitudinal axis of the future animal. 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. . 



