40G DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



cleavage becomes increasingly irregular, but there is a tendency, 

 as long as the relationships can be determined, for each cleavage 

 plane to appear at right angles to that which has preceded it, as in 

 many other cases of cell division. The principal factor that mod- 

 ifies this ideal relationship is the mutual pressure which tends to 



Fig. 214. — Early cleavage, blastula, and gastrula stages of the frog. A, the 

 zygote or one-cell stage. B, two-cell stage with second cleavage furrow in 

 proccds of development. C, fight-cell stage. D, late blastula in section, 

 nuclei not shown. E, early gastrula in section corresponding with future 

 median plane of body; direction of more active growth marked by arrows. 

 F, late gastrula showing further development of archenteron; direction of 

 more active growth indicated by arrows as in E. 



bp, blastopore; e, archenteron; p.b., polar bodies; sc, segmentation cavity or blastocoele. 

 {E and F, redrawn from McEwen, "Textbook of Vertebrate Embryology," copyrignt, 1923, 

 by Henry Holt and Co., printed by permission.) 



make the cells assume particular outlines as do the units in a mass 

 of soap bubbles. Thus, the two fourth cleavage planes by which 

 the twelve- to sixteen-cell stage is formed, are in the polar axis, 

 and therefore lie at right angles to the third {cf. Fig. 214 C). They 

 consist of two planes, which may cut through the poles at right 

 angles to each other or form a more irregular pattern, and which 



