MATURATION OF THE EGG 



31 



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

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

 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 Amcebw. 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 



i Y\7"^ 1, -> tozoa. . Of a Mammal ; b, of a Turbel- 



and produce spermatozoa. We have larian> wi ; h two accessor ' y fl a ,, ella . Cj t?j 

 already seen a phenomenon similar, ;m d e, of Nematoda; /, of a Crustacean; 



though not in all points parallel, in .". of a Salamander (with undulating mem- 

 ,, -. ,. 11 r ii_ 1 brane);7i, The commonest pin-sliapedform. 



Volvox. An ordinary cell of the colony 



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



spermatozoa. 



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. 27. Various forms of Sperma- 



A 



B 



- to 



- P n \ 



- nu \ 



----;->... sp 



" 



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

 eggs of Asterias glacialis, after O. Hertwig. .r, Prominence of protoplasm ; /,/, germinal spot 

 which divides into two distinct substances, pn and nu ; 7.-7j, germinal vesicle ; up, nuclear spindle. 



considerable changes. It becomes partially disorganised (Fig. L'S). 

 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 



