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MANUAL OF ELEMENTARY ZOOLOGY 



white, yolky portion. The second division is at right 

 angles to the first and forms four similar blastomeres ; 

 the third division is horizontal and separates four small, 

 pigmented upper blastomeres from 

 four large, yolky lower blastomeres. 

 By succeeding divisions sixteen and 

 then thirty-two blastomeres arise, after 

 which cleavage becomes irregular, the 

 pigmented cells dividing more rapidly 

 than the yolky. The final result is 

 the formation of a blastula, in which 

 the floor of the blastoccele is composed 

 of large yolky cells and the roof of 



Fl °- jSrTof atg SmaU Pigment , ed CdlS - * *? **" 

 egg ai the end of the upper cells merge gradually into 



cieavage. the lower. The pigmented cells are 



He, Biastocrcie. the future epiblast, the yolk cells will 



give rise to the hypoblast, and both 



regions differ from the corresponding parts of the blastula of 



Amphioxus in being more than one cell deep, though the 



white layer is much thicker than the black. From this 



blastula a gastrula is formed, not by invagination, which 



would be impossible on account of the relative amount of 



l.bl. l.bl. l.bl. l.bl. 



A B C D 



FlG. 368. — Stages in the gastrulation of the frog's egg. The egg is 

 seen from the lower, white pole, which faces towards the future 

 hind end of the animal. 



/./'/., T,ip of the blastopore : v./., yolk plug. 



the two layers, but by an overgrowing of the epiblast over 

 the yolk cells. 



When this process begins, the black epiblast and the white yolk 



, a cells each form half the outer surface of the blastula, 



Dastrulatlon. , ■ , ,, , ,1,1 111 j tm 



whirli floats with the black side uppermost. The 



epiblast grows downwards over the surface ol the yolk rells, narrowing 



