PAET HI. THE DEVELOPMENT OF 

 VEETEBEATA. 



CHAPTER XVII. GENERAL AND AMPHIOIUS. 



1. The Ovum. The ripe egg or ovum of any animal, as 

 found in the ovary, is a single nucleated cell (fig. 110). In 

 Vertebrates it is always surrounded by a follicle, which in 

 the simplest case is a single layer of small cells j and within 



this is a vitelline 

 membrane, which is 

 a secretion, probably 

 of the ovum itself. 

 Although it varies in 

 size enormously in 

 different animals 

 (from -g-ig- of an inch 

 in diameter in the 

 case of Amphioxus to 



Fig. 110. AN OVUM. v J.T. 



over an inch in the 



case of the fowl), its nucleus (or germinal vesicle, as it is 

 customary to call it) does not vary proportionately, but is 

 always microscopic (though relatively large). The differ- 

 ences in size are, in fact, almost entirely due to the varying 

 quantity of food-material or yolk which is stored within the 

 protoplasm of the cell. This yolk is an endoplastic product 

 of the ovum, destined in the course of development to be used 

 up in the formation of new protoplasm. Small ova are ova 

 with little or no yolk alecithal they are technically called. 

 In such cases we usually find that the developing individual 

 becomes active and capable of feeding in regular animal 

 fashion at a comparatively early stage in its development, 

 while it as yet differs greatly from its parent : it is then 

 called a larva. On the other hand, the young individual 



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