GENERAL OUTLINE 



235 



of the four cells forms the " ovum " proper, while the other three, 

 known as the polar bodies, are minute, rudimentary, and incapable of 

 development (Figs. 89, 97, 114). In the male, on the other hand, all 

 four of the cells become functional spermatozoa. This difference 

 between the two sexes is probably due to the physiological division of 

 labour between the germ-cells, the spermatozoa being motile and very 

 small, while the ^gg contains a large amount of protoplasm and yolk, 

 out of which the main mass of the embryonic body is formed. In the 

 male, therefore, all of the four cells may become functional ; in the 

 female the functions of development have become restricted to but one 



Primordial germ-cell. 



Oogonia. 



Primary oocyte or ovarian egg. 



Secondary oocytes (egg and 



first polar body). 



Mature egg and three polar bodies. 



■ Division-period (the number of divi- 

 sions is much greater). 



Growth-period. 



. Maturation-period. 



Fig. 115. — Diagram showing the genesis of the egg. [After BOVERI.] 



of the four, while the others have become rudimentary (r/. p. 124). 

 The polar bodies are therefore not only rudimentary cells (Giard, '76), 

 but may further be regarded as abortive eggs — a view first put forward 

 by Mark in 1881, and ultimately adopted by nearly all investigators.^ 

 The evidence is steadily accumulating that reduction is accomplished 

 by two maturation-divisions throughout the animal kingdom, even in 

 the unicellular forms ; though in certain Infusoria an additional divi- 

 sion occurs, while in some other Protozoa only one maturation-division 

 has thus far been made out. Among plants, also, two maturation- 



1 A beautiful confirmation of this view is given by Francottes's ('97) observations on a 

 turbellarian, Prosthecerceus. The first polar body is here often abnormally larj^c, all grada- 

 tions having been observed from the normal size up to cells nearly as large as the egg itself. 

 Such polar bodies are occasionally fertilized ^nA develop into small gastrulas, first forming a 

 single polar body like the second polar body of the egg. Here, therefore, two of the four 

 cells are exceptionally capable of development. It may be added that Fol long ago observed 

 the penetration of the small polar bodies by spermatozoa in the echinoderms; and this has 

 been more recently observed by Kostanecki in mollusks. 



