EMBR } 'ONIC DE I 'EL OPMENT. 



107 



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 

 marks 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 

 blastomeres is followed by an- 



nt-h^r nnf > Pt rJo-Kr nn crlpc tn it Fig ' 55- E 'g ht - ce11 sta g e seen 



right angle it, from the U p per (animal) pole . Four 



dividing each Of the tWO blastO- sma11 blastomeres (micromeres) lie 



upon four larger blastomeres fma- 

 mereS again into tWO. In this cromeres). Radial type of cleavage. 



way the stage with four equal (After E. B. WILSON.) 

 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- 



