XII 



DEVELOPMENT 



201 



the white until only a small circular space the blastopore, 

 filled by a mass of yolk-cells the yolk-plug, is left 

 uncovered (G, H, yk. pi). 



Development of the Chief Organs. In the mean- 

 time a second cavity appears in the interior of the 

 embryo, and as it increases in size, the segmentation- 

 cavity undergoes a proportional diminution and finally 

 disappears. This new cavity, or enteron(Figs. 64, 1, and 65 



mes 



FIG. 65. Transverse sections through the body of a Frog embryo. A, with open 

 medullary groove (compare Fig. 64, J). B, with closed medullary groove 

 (compare Fig. 64, L.) - ( x about 30.) 



c^coel. coelome; ccel 1 , prolongation of ccelome into mesodermal segments (pr. v); 

 ect. ectoderm ; end. endoderm; ent. enteron; mes, msd. mesoderm ; mes 1 ,sotn. 

 outer paristal, and mes 2 , spl. inner visceral layer of mesoderm ; md. f. medullary 

 fold; md. gr. medullary groove; nch. notochord ; pr. v. mesodermal segment; 

 sg. d. primary kidney duct (pronephric duct) ; sp. c. medullary cord; yk. yolk- 

 cells. ( \fter Marshall.) 



ent), is the rudiment of the enteric canal : it begins to be 

 formed as a narrow slit at the edge of the blastopore 

 through which it at first communicates with the exterical 

 but it soon becomes closed : at present there is neither 

 mouth nor anus. 



By this time the cells of which the embryo is com- 

 posed have assumed different forms and become arranged 

 in a very definite manner. First of all there are several 

 layers of small pigmented cells, derived from the black 



