MATURATION OF THE EGG 



31 



cell nucleus, and a mobile filamentous appendage, the tail, whicli is of 



protoplasmic nature, and is much like the flagellum of the Fkgellata. 



Besides the tail there may be accessory flagella. Between the head and 



the tail a special intermediate portion is sometimes interposed. Other 



forms of spermatozoa are occasionally 



found; round, pear-shaped, etc., either 



stationary or moving like Amcehce. The 



spermatozoa arise in the testis from a 



germinal layer or epithelium, as do 



eggs. After repeated division of the 



original formative elements, cells are 



produced which are equivalent to egg 



germs, and which may be distinguished 



as sperm germs. Whereas, however, 



the egg germs become eggs direct by 



means of growth and maturation, the 



sperm germs are still further divided 



and produce spermatozoa. 



abeady seen a phenomenon similar, 7^1 e, ot Nematoda ; f, ot ,1. Crustacean 



though not in all points parallel, in ff, of a SaZamajiSer (with undulating mem- 

 VOIVOX. An ordinary cell of the colony ^rane); ft, The commonest pin-shapedtorm. 



there becomes by growth a large egg, or by division a mass of small 

 spermatozoa. 



Fig. 27.— Various forms of Sperma- 

 We have ^°^°^- "■ ^^ ^ Mammal ; 6, of a Turbel- 

 larian, with two accessory flagella ; c, d. 



Maturation of the Egg. 



The ejection of the directive or polar bodies is the last stage in 

 the maturation of the egg which precedes fertilisation. The germinal 

 vesicle moves towards the surface of the egg (towards the animal pole 

 in the case of eggs differentiated into poles), and here undergoes 



Fig. 28.— Disorganisation of the germinal vesicle and formation of the nuclear spindle in 

 eggs of Asterias glaciaUs, after O. Hertwlg. x, Prominence of protoplasm ; kf, germinal spot 

 whicli divides into two distinct substances, pn and nu ; kh, germinal vesicle ; sp, nuclear spindle. 



considerable changes. It becomes partially disorganised (Fig. 28). 

 Out of part of its contents is formed that spindle-shaped figure 

 (Fig. 28, B) which is characteristic of indirect nuclear division (see 

 below, pp. 35,36). The one half of the spindle enters a small mass of 

 protoplasm which projects from the surface of the egg. This prominence 



