THE GERM-CELLS 



33 



(&) OVUM. 



Coming now to the description proper of the germ-cells, 

 we shall start with the Ovum. 



The ovum is a cell, and has as such the typical cell- 

 structure. It has a cell-body, limited by a cell-membrane, 



.2k 



R 



FIG. 22. OVUM' OF SEA-URCHIN, TOXOPNEUSTES LIVIDUS. (After 



Wilson.) 



zk, cell-body ; k, nucleus ; n, nucleolus. Below the ovum the 

 spermatazoon (sp) of the same animal is drawn with the same 

 magnification. 



and, further, a nucleus, here called the " germinal vesicle/' 

 which again contains the nucleolus and the chromatin 

 substance. 



The size of the ovum varies considerably. It is often 

 microscopical, as in the mammalian eggs, but can attain, 

 on the other hand, enormous proportions, as in the case of 

 birds. The essential part of the egg-cell, however the 



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