REPRODUCTION BY GERM-CELLS 



281 



x&mv 





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••''j-^.'-S-.' /"■,'■ ••..• .' I. ' 







endoderm and finally return to quite definite and often remote 8 p 

 m the ectoderm (Eudevdrium, Fig. 95 ). In another hydroid 3 

 (Corydendr^um parasiticus the mature egg-cells leave their former 

 pos^on withm the endoderm and creep entirely outside of the animal 



winch produced them, establishing themselves in a definite spot 0D 

 external surface, where they await the fertilizing zoosperms Manx- 

 ova can accomplish slight amoeboid movements, but in tnosl animafc 



these do not suffice for movement from place to place, and tl VB 



remain quietly in the 

 spot where they were 

 developed, or are pas- 

 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 

 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; fchus the ova 

 of the brown sea- wracks, or Fucaceae, for instance, arc quite twenty 

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

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

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

 the embryo from the mother-plant is excluded. 



Fig. 69. Ovum of the Sea-urchin, To.xopneusics lividus, 

 after Wilson, sk, cell-body. A-, nucleus or so-called 



' germinal vesicle.' 



nucleolus or so-called 'ger 



minal spot.' Below there is a spermatozoon <>f the 

 same animal, with the same magnification (750 

 times). 



