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EMBRYOLOGY 



DEVELOPMENT OF THE LANCELET 



Hitherto we have been concerned almost wholly with the 

 anatomy and physiology of adult animals. We must now give 

 some attention to the process by which the adult arises from the 

 fertilised egg. For this purpose we shall study first the development 

 of the lancelet, which is relatively simple and easy to follow 

 owing to the fact that the protoplasm of the ovum is not hampered 

 with a large amount of yolk. The egg is about o'l mm. in diameter, 

 and secretes for itself a thin covering, the vitelline membrane, 

 which will become more distinct when the cytoplasm shrinks from 

 it after fertilisation (p. 5). Before it is laid, the egg undergoes a 

 very unequal division, the first maturation division, in which half 

 the nuclear material and a very small amount of cytoplasm are 

 separated as the first polar body. The second maturation division, 

 with the formation of the second polar body, occurs just after 

 fertilisation. The nuclear changes which take place in these 

 divisions are described in Chapter 29. The ovum is one of a 

 kind in which there can already be distinguished portions of the 

 cytoplasm which are destined to form particular regions of the 

 future body, though in Branchiostoma these are not yet in position. 

 Under the vitelline membrane is a layer of cytoplasm which is 

 finely granular but free from yolk granules. Within this the cyto- 

 plasm contains yolk granules. A large nucleus (germinal vesicle) 

 displaces it from a good deal of one side of the ovum. This is the 

 animal pole ; the yolky side is the vegetative pole. The animal pole 

 is usually spoken of as the upper side, but actually when the embryo 

 is formed this side will be ventral and at the front end, while the 

 vegetative side will be dorsal and behind. When the egg is fertilised 

 certain changes occur. In preparation for the union of the two 

 sets of chromosomes, one from each gamete nucleus, which takes 

 place near the middle of the egg, the nuclear membrane of the 

 germinal vesicle breaks down and a quantity of clear cytoplasm 

 takes the place of the vesicle. Meanwhile the granular cytoplasm 

 which has covered the ovum flows to the vegetative pole and there 

 becomes a crescent around the future posterior end. The clear 



cytoplasm which now forms the animal side of the egg will give 



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