EMBR YOXIC DETELOPMEKT. 



1 07 



brane is the first outward and visible sign of the accom- 

 plishment of the process of fertilisation. 



About an hour later, — that is to say, at about 8 p.m., — 

 the egg becomes flattened at its two poles, and a depression 



Fig. 54. — Division of ovum into the first two blastomeres. The polar body 

 marlis the animal pole. (After Hatschek.) 



appears at the animal pole, the latter being indicated by 

 the polar body. The depression deepens and extends as a 

 meridional furrow round the egg. Finally, the division of 

 the egg into two halves or blastomeres, which remain at- 

 tached to one another, is completed, and the first stage in 

 the segmentation of the egg is accomplished (Fig. 54). 



As it is beyond the scope of 

 this book to discuss the mechan- 

 ics of cell-division, the descrip- 

 tion of the segmentation stages 

 will be very brief. 



The first meridional cleavage 

 which divides the egg into two 

 blastovicrcs is followed by an- 

 other one at right angles to it, 

 dividing each of the two blasto- 

 meres again into two. In this 

 way the stage with four equal 

 blastomeres in one plane is produced. Next follows an 

 equatorial cleavage, by which eight blastomeres are pro- 

 duced, the four upper cells at the animal pole being some- 



Fig. 55. — Eight-cell stage seen 

 from the upper (animal) pole. Four 

 small blastomeres (micromeres) lie 

 upon four larger blastomeres (ma- 

 cromeres). Radial type of cleavage. 

 (After E. B. VViLSiiN.) 



