xvii PROTOCHORDATA 611 



what will be the animal pole of the egg. In the centre of this cap 

 lie the chromosomes which are connected together by a faintly 

 granular substance. Out of this granular substance the first matura- 

 tion spindle is developed, the spindle fibres appearing as rods of 

 granules. No further development takes place unless the egg be 

 fertilized. 



The spermatozoon is able to burrow through the chorion ; it 

 enters the cytoplasm at the vegetative pole of the egg, as in the case 

 of the egg of Amphioxus. Immediately after its entry the division 

 initiated by the maturation spindle is completed, and the first polar 

 body is given off. This consists of clear cytoplasm and does not 

 divide. A new spindle is reconstructed out of the remains of the 

 old one, and a new division occurs by which the second polar body is 

 given off. The second polar body is in all respects similar to the 

 first ; both 1 todies remain attached to the egg, in many cases through- 

 out the whole of the embryonic development, and so the position of 

 the animal pole can be accurately determined. In neither matura- 

 tion division is there a trace of a centrosome. 



Whilst the maturation divisions have been proceeding, cytoplasmic 

 movements of a violent character have been taking place. Both the 

 clear cytoplasm and the clear peripheral layer with the yellow 

 granules flow downwards to the vegetable pole of the egg, and form 

 a cap there which surrounds the sperm head. With such violence 

 does this flow take place that the " test cells " are often carried along 

 with it, and are found aggregated in a heap at the vegetable pole 

 (Fig. 442, B). At the animal pole is left only a small mass of clear 

 substance surrounding the maturation spindle. The slaty yolk is 

 now massed at the animal pole. At the vegetable pole the clear 

 protoplasm is superficial to and. more extensive than the yellow 

 substance. 



The spermatozoon at first moves towards the centre of the egg 

 from whatever point it enters, until it has traversed the clear and the 

 yellow protoplasm and reached the yolk. This is its penetration 

 path. Then it changes its direction and moves towards the posterior 

 end of the egg. This is its copulation path. As it thus moves it 

 seems to draw with it a large part of the yellow protoplasm, which 

 forms a crescent just below the equator of the egg, with the two 

 arms reaching half-way round ; the centre of this crescent is the lower 

 pole of the egg. The clear cytoplasm moves towards the posterior side 

 of the egg, around and outside the yellow substance (Fig. 442, C). 



The male and female pronuclei meet near the lower pole of 

 the egg ; since the female pronucleus, after giving off the second 

 polar body, descends to meet the male. The zygote nucleus then 

 moves inwards towards the egg's centre. The male pronucleus, 

 before uniting with the female pronucleus, has developed at out- 

 side a well-marked aster. Before the zygote-nucleus divides, this 

 aster divides into two parts, placed one at each end of the first 

 spindle ; so that, although in the maturation divisions no asters are 



