JlMBRYOXIC PERIOD. 49 



1. The Egg. 



The ripe egg of Ampbioxus is a spherical mass of proto- 

 plasm, 0-105 mm. in diameter on the average, and inclosed in 

 an elastic vitelline membrane. The protoplasm is studded with 

 numerous yolk granules, which are sufficiently opaque to hide 

 the nucleus. At one pole, which will be spoken of as the upper 

 .pole, there is a slightly flattened patch of protoplasm compara- 

 tively free from yolk granules ; and on the top of this patch is 

 a sharply denned polar body (Fig. 14, I, PB). A second polar 

 body has not been seen. 



The vitelline membrane, prior to fertilisation, adheres closely 

 to the egg. 



2. Fertilisation. 



The male Amphioxus, as described above (p. 46), sheds 

 spermatozoa over the eggs as these are laid by the female ; 

 and they may be seen adhering in numbers to the vitelline mem- 

 branes. The details of fertilisation have not been studied, but 

 shortly after the spermatozoa gain access to the egg the vitelline 

 membrane, which previously invested the egg closely, swells up 

 rapidly by imbibition of water, and becomes separated from the 

 egg by a considerable space; the egg ultimately lying in the 

 centre of a capsule three or four times its own diameter. The 

 purpose of this swelling up of the vitelline membrane, and its 

 separation from the egg, is probably to prevent the entrance ol 

 other spermatozoa after the egg has been fertilised. 



3. Segmentation. 



The process of segmentation commences at dusk, usually 

 about 8 P.M., and is completed in about three hours. 



The first cleft appears about an hour after the eggs are laid 

 and fertilised. It commences as a depression at the upper pole of 

 the egg, close to the polar body, extends rapidly across the upper 

 pole, and then spreads quickly round the egg as a groove (Fig. 14, 

 n). The groove deepens rapidly, being always more prominent at 

 the upper than the lower pole ; and in about five minutes from 

 its first appearance the cleft is completed, the egg being divided 

 by it into two halves or blastomeres of equal size, and similar in 

 all respects save for the presence of the polar body on the apex 

 of one of them. 



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