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Elementary Biology. 



and comparable to the outer layer of the gastrula in Am- 

 phioxiis, the large enclosed cells being comparable to the 

 inner layer of the gastrula, though they do not take the 

 same share in the formation of the alimentary canal as in 

 the case of Amphioxus. The small segmentation cavity 

 also is left for a time between the two layers in the frog 

 embryo. This, however, soon becomes obliterated. The 

 blastopore is present in both, though formed rather as a 



FIG. 186. FORMATION OF POLAR BODIES FROM THE UNFERTILISED OVUM 

 (Quain.) 



i, segmentation of nucleus and formation of nuclear spindle, g.v. ; 2, 3, 

 extrusion of first polar body,/' ; 4, 5, extrusion of two polar bodies and 

 formation of female pronucleus, //. 



consequence of the activity of the epiblast than as a result 

 of the sinking in of the hypoblast. It is plugged up in the 

 frog. 



Before the epiblastic cells have enclosed the yolk-cells 

 a ridge or lip of epiblast arises at the margin of the blastopore, 

 and, pushing its way backward, roofs over a cavity whose 

 floor is formed by the yolk-cells. This cavity is the 

 mesenteron, and the layer of epiblastic origin which forms 



