REPRODUCTION BY GERM-CELLS 



281 



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

 in the ectoderm {Eudendrmm, Fig. 95). In another hydroid polyp 

 (Corydendrium 2^arasiticum) the mature egg-cells leave their former 

 position within the endoderm and creep entirely outside of the animal 

 which produced them, establishing themselves in a definite spot on its 

 external surface, where they await the fertilizing zoosperms. Many 

 ova can accomplish slight amoeboid movements, but in most animals 

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

 remain quietlj^ in the 



..■.■■i'iry'V.vvVV''v-« 'y /■ 



.w<Vi^/■i^>■'•-^;^■'^■'^'V^''>i^i^>^•-.;>'' 



spot where they were 

 developed, or are j^as- 

 sively pushed to another. 

 Cases such as that of the 

 polyp I have cited, in 

 which the ovum actually 

 comes to meet the male 

 element, are quite excep- 

 tional, for in general 

 the ovum is the passive 

 and the spermatozoon 

 the active or exploring 

 element in fertilization. 

 The female cell is en- 

 trusted with procuring 







■■'■^•^^@S4§,>ii^-''^'" 



'/" 



Fig. 69. Ovum of the Sea-urchin, Toxopneustes lividus, 

 after Wilson, sk, cell-body, k, nucleus or so-called 



nucleolus or so-called ' eer- 



' germinal vesicle.' 



minal spot.' Below there is a spermatozoon of the 

 same animal, with the same magnification (750 

 times). 



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, of ten 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 Fucacese, for instance, are quite twenty 

 times as large as the ordinary cells of the alga3 (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. 



