92 OUTLINES OF CHORDATE DEVELOPMENT 



some lots nearly one-half, of the eggs show some departure 

 from this arrangement, and the third cleavage may be horizon- 

 tal in only one, two, or three blastomeres, and vertical in the 

 remainder or, rarely, vertical in all four. These and also the 

 later vertical planes frequently do not pass actually to the upper 

 pole of the egg, and are therefore not strictly meridional, 



FIG. 30. Cleavage of the frog's egg. After Morgan. Animal pole upward 

 in all figures. (For earlier stages see -Fig. 27.) A. Eight cells. B. Twelve cells 

 becoming sixteen. C. Thirty-two cells. D. Forty-eight cells, more regular than 

 usual. E, F. Posterior and anterior views of about 128 cell stage. G. Late 

 cleavage or early blastula. H. Commencement of gastrulation (cell outlines 

 indicated only in the region below the invaginating groove), i, Beginning of 

 invagination. 



although there is a decided tendency for them to lie in me- 

 ridians. The location of the succeeding cleavages varies with 

 that of the third. Typically the fourth cleavages (Fig. 30, B) 

 are meridional forming eight small upper, and eight large lower 

 cells, and the fifth again latitudinal, forming thirty-two cells 

 arranged in four horizontal rows of eight cells each (Fig. 30, C). 

 But the atypical appearance of some of the third cleavages may 

 very early disturb this schema. With the appearance of the 



