228 



THE CELL 



smaller cell, richer in protoplasm, and a larger one, richer in 

 yolk; in these segmentation proceeds at different rates. 



Ic. Cell-Budding. 



When one of the portions produced by division is so much 

 smaller than the other, that it appears as though it were only 

 a small appendage to the original cell, scarcely causing any 

 diminution of its substance, the process is called "cell-budding, or 

 gemmative segmentation," the smaller portion being called the 

 bud, and the larger the mother-cell. Two kinds of cell-budding 

 are distinguished, according to whether one or more buds are 

 formed. 



In the animal kingdom this process of cell-budding occurs when 

 the egg is mature, causing the development of the directive cor- 

 puscles, or polar bodies (polar cells). By this term we understand 

 two or three small spherules, which are composed of protoplasm 

 and nuclear substance, and hence are of the same value as small 

 cells ; they are frequently situated at the animal pole of the egg, 

 within the vitelline membrane. The course of the process of cell- 

 budding is as follows : 



Whilst the germinal vesicle is becoming broken up, a typical 



II. 



III. 



IV. V. VI. 



Fig. 118. Formation of the polar cells in Asterias glacialis (O. Hertwig, EmbrxjoL, Fig. 

 13). In Fig. I. the polar spindle (sj>) has advanced o the surface of the egg. In Fig. IT. 

 a small protuberance (rk l ) has been formed, which receives halfof the spindle. In Fig. III. 

 the protuberance is constricted off, forming a polar cell (rfc 1 ). Out of the remaining halfof 

 the original spindle, a second complete spindle (.sp) has developed. In Fig. IV. a second 

 protuberance has bulged out below the first polar cell, which in Fig. T'. becomes constricted 

 off to form the second polar cell (rife 8 ). In Fig. VI. out of the remainder of the spindle the 

 egg nucleus (ek) develops. 



