DEVELOPMENT OF THE BOG-FISH. 



THE OVUM 



Is a minute oval body produced in the ovary — it consists of the 

 following parts : — 

 (1.) A yolk mass. 

 (2.) A small cell lying on the surface^ 



This cell is bounded by a dark outline, the vitelline membrane 

 and embedded in it is the germinal vesicle, inside of which again 

 is the germinal spot. The germinal vesicle and spot represent, 

 respectively the nucleus and nucleolus. 



The germinal vesicle and spot disappear before the ovum 

 leaves the ovary, this being independent of fertilization. Impreg- 

 nation consists in the entrance into the cell of a spermatozoon. 

 The spermatozoa are the male elements produced from the 

 testes. Each spermatozoon consists of a pointed head with a long 

 tail, and is actively motile, which property is due to the lashings of 

 the tail, After fertilization, the egg on its way down the oviduct 

 receives a new investment of an albuminous material. After 

 impregnation, a furrow appears on the surface of the cell, which, 

 gradually deepening, at length cuts into two : while the- same 

 process begins almost immediately in each half, and cuts it also 

 into two. The same process is repeated in each of the quarters,; 

 and so on, until at last, by continued cleavings, the, whole yolk 

 is converted into a mulbery-like mass, the whole still enclosed 

 in the vitelline membrane. From this it is seen that only a 

 portion of the ovary undergoes segmentation, and is therefore 

 ■called meroblastic (i.e., part only undergoing division). 



The ova of all fishes, birds, and reptiles are meroblastic. On 

 the other hand, in the mammalia and amphibia the entire ovum 

 undergoes segmentation, and is hence termed holoblastic (i.e., 

 whole dividing). The ovum of Amphioxus is likewise holoblastic. 

 When the peripheral cells, which are first formed, are fully 

 developed, they arrange themselves into a kind of membrane' 

 at the surface of the yelk, at the same time assume a polyhedral 



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