VERTEBRATE DEVELOPMENT 225 



cells results in an actual covering up of the light-colored cells of the 

 vegetal pole so that as the process proceeds, the area of the vegetal 

 pole as seen externally constantly diminishes, until finally only a 

 tiny portion, the yolk plug, remains. (W. f. 174, G.) 



Coincident with the overgrowth there is a turning under, or 

 invagination, of the outer dark ectoderm cells to form the two- 

 layered gastrula. This process begins in a small area (gray cres- 

 cent) lying near the boundary between the dark and white cells. 

 The area of invagination soon becomes circular in outline, and then, 

 as the growth over the white cells continues, contracts in diameter, 

 finally reaching the stage noted above, where only a tiny area of 

 the white cells, the yolk plug, is visible externally. The circular 

 area of invagination is known as the blastopore. It indicates the 

 posterior end of the embryo and the approximate position of the 

 future anal opening. (W. f. 174, /.) 



Inside the embryo at this stage, a considerable amount of endo- 

 derm has formed near the blastopore. It is thickest on the dorsal 

 side but spreads laterally, soon forming a definite layer beneath the 

 ectoderm. It obliterates the original blastocoel and a new endo- 

 dermal-lined cavity is formed, which constitutes the primitive ali- 

 mentary canal, or enteron, of the embryo. It terminates pos- 

 teriorly at the blastopore which, as mentioned above, marks 

 approximately the position of the permanent anal opening. The 

 mouth opening is later formed at the opposite end of the develop- 

 ing embryo. During gastrulation the mesoderm is also forming 

 in the region of the blastopore as a layer between the ectoderm 

 and endoderm. The ectoderm, endoderm, and mesoderm con- 

 stitute the primary germ layers from which all the tissues and 

 organs of the Frog develop. (W. f. 174, H.) 



During the process of invagination, a median longitudinal strip 

 of cells lying above the enteron begins to differentiate and to be- 

 come separated from the mesoderm. From it an axial rod of 

 tissue, the notochord, develops. This is the forerunner of the 

 segmented, bony vertebral column which later develops around 

 the notochord. An external examination of an egg at about this 

 stage, when the blastopore is nearly closed, shows a differentiation 

 of the ectoderm on the dorsal side of the body to form the medul- 

 lary plate which is the forerunner of the central nervous 

 system. The edges of the medullary plate, on either side of the 

 median line of the embryo, become enlarged and somewhat ele- 



