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AN INTRODUCTION TO ZOOLOGY 



shifts slightly, bringing about a rotation, so that the blastopore 

 comes to lie well above the equator. It is visible for some time and 

 is filled with a small group of the white vegetative cells, which is 

 termed the yolk plug. While these external changes have been 

 taking place no less important ones have occurred internally. The 



FIG. 132. Rana, sections. From Marshall. 



The four small figures represent the segmenting egg, seen from its lower pole, and illustrate 

 the formation and shifting of the blastopore (BP.). The four large figures are sections of the egg 

 taken vertically through its horizontal axis Y.Z., and represent four stages in the formation of the 

 mesenteron. Notice the rotation of the egg through more than go". 



D.L., dorsal lip of blastopore ; I.C., intermediate cells ; MN., mesenteron ; SC., segmentation 

 cavity or blastoccel ; V.L., ventral lip of blastopore ; WX., axis, of embryo, which is at first vertical 

 and subsequently antero-posterior ; YZ., axis of embryo, which is at first horizontal and subse- 

 quently vertical or nearly so. 



vegetative cells push their way up as a moderately thick layer 

 underneath the dorsal lip of the blastopore which becomes turned 

 in and moves slightly with them. They move round the dorsal 

 side of the segmentation cavity, leaving a small irregular cleft 

 between themselves and the ectoderm cells growing over them, 

 and finally they reach the yolk cells of the opposite or anterior end. 



