REPRODUCTION BY GERM-CELLS 



281 





m 



0m. 



endoderm, and finally return to quite definite and often remote spots 

 in the ectodorni (Eudmdrmm, Fig. 95). In another hydroid polyp 

 (Corydendrium parasiticum) the mature egg-cells leave their former 

 position within the endoderm and creep entirely outside of the animal 

 which produced ihcni, estahliwhing themaelvea in a definite spot on its 

 externul surface, whore they await the fertilizing zoosperms. Many 

 ovii can accomplish slight auKcboid movements, but in most animals 

 thesis do not suffice for movement from place to place, and the ova 

 I'oniain quietly in the 

 spot where they were 

 developed, or are pas- 

 sively pushed to another. 

 Casus such as that of the 

 polyp I have cited, in 

 which the ovum actually 

 comes to moot the male 

 element, are quite excep- 

 tional, for in general 

 the ovum is the passive 

 and the speruiato^soon 

 the active or exploring 

 element in fertilization. 

 The female cell is en- 

 trusted with procuring 

 and storing the material 

 necessary to the building 

 up of the embryo ; and 

 its peculiarities depend 

 chiefly on this. • 



It is true that in 

 plants this stored material 



is seldom considerable, and that is because the ovum so frequently re- 

 mains even after fertilization within the living tissues of the plant, and 

 is thence supplied, often very abundantly, with food-stuffs; and, more- 

 over, because the young plant that springs from the fertilized ovum may 

 be very small and simple, and yet capable of immediately procuring 

 its own nourishment. But there are exceptions to this ; thus the ova 

 of the brown sea-wracks, or Fucaceas, for instance, are quite twenty 

 times as large as the ordinary cells of the algaj (Fig. 64), and contain 

 a quantity of food-stuff within themselves. In this case the ova are 

 liberated into the water even before fertilization, and the nutrition of 

 the embryo from the mother-plant is excluded. 





'p 



Pio. 69. Ovum of tlio Sea-urchin, roiopnrosfes lividus, 

 after Wilson, ck, ooll-body. k, nucleus or so-called 

 ' germinal vesicle.' n, nucleolus or so-called ' ger- 

 minal spot.' Below there is a spermatozoon of the 

 same animal, with the same magnification (750 

 times). 



