34 



THE DEVELOPMENT OF THE CHICK 



Thus at each end of the spindle there are eight dyads. Those at 

 the outer end then enter a Uttle bud of protoplasm projecting 

 above the surface of the germinal disc, and this bud with the 

 dyads is cut off as the first polar body, which lies in a depression 

 of the germinal disc beneath the vitelline membrane (Fig. 11). 

 Eight dyads, therefore, remain within the germinal disc. 



A second maturation spindle is then formed almost imme- 

 diately, apparently without the intervention of a resting stage 

 of the nucleus, and takes a radial position similar to that occupied 

 by the first, with the dyads forming an equatorial plate (Fig. 11). 





^■-v.M. 



t.^sm 



'71. 3 p. 2. 





Fig. 11. — Second maturation spindle and first polar hody of the pigeon's 

 egg; a combination of two sections. 8.15 p.m. x 2000. (After Harper.) 

 m. Sp. 2, Second maturation spindle, p. b. 1, First polar body. v. M., 

 Vitelline membrane. 



Each dyad then divides along the preformed plane of division, 

 and the daughter-chromosomes diverge towards opposite poles 

 of .the spindle. The outer end of the second maturation spindle 

 then enters a superficial bud of the protoplasm of the germinal 

 disc similar to that of the first maturation spindle; and this bud 

 together with the contained chromosomes becomes cut off as the 

 second polar body. 



The result of these processes of maturation is the formation 

 of three cells, viz., the two polar bodies and the mature egg. 

 The polar bodies are relatively very minute and soon degenerate 

 completely. 



After the formation of the second polar body there remain 

 in the egg eight chromosomes, each of which represents one 

 quarter of an original tetrad. These form a small resting nucleus 

 known as the egg-nucleus or female pronucleus. It is many 

 times smaller than the original germinal vesicle (Fig. 12), and 



