194 



GENERAL BIOLOGY 



blastopore, but, instead, a narrow crescentic slit, the edges of 

 the blastopore being pressed together as shown in figure iigi. 

 Then the edges of the crescent extend downward until they 

 meet below a little circular patch of protruding yolk, the yolk 

 plug. Except upon the upper side where lies the direct 

 entrance to the archenteron, they cut but a shallow circular 



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m 



n 



Fig. 119. Early development of salamander eggs, a to h, i-. 2-, 

 4-, 8-, t6-, 32-cell and later segmentation or cleavage stages; ?. 7, gas- 

 trulation stages; i, shows the crescentic blastopore and /, the fully formed 

 yolk plug;fe to m, formation of the neural tube. 



groove upon the surface of the yolk. If one conceive of his 

 own head as the sphere of the gastrula salamander, his 

 closed mouth the blastopore, and the corners of his mouth 

 pulled down until they join beneath his chin, he will get a 

 clear conception of the relation of these parts. Figure 121 

 shows in longitudinal sections the formation of the gastrula, 



