The Development of Phascolosoma. 91 



seen evidence that the centrosome within this astrosphere at any 

 time divides. The latter at this time is essentially a large vacuole, 

 containing a centrosome and surrounded by prominent cytoplasmic 

 radiations. It bears evidence that a difference in osmotic pressure 

 has existed between it and the surrounding protoplasm, in that it 

 has absorbed cell sap even faster than the egg nucleus, as shown 

 by its excessive size. It is possible that this osmosis may play 

 some part in the final migration together of the two pronuclei. 



The movement of the sperm nucleus towards the active pole 

 results in a flattening of the astrosphere (Fig. 18). The astrosphere 

 and pronucleus now rotate around each other, until they lie side 

 by side, about equidistant from the active pole of the egg. At this 

 time the sperm astrosphere (Fig. 19, ast. s) becomes reduced in size 

 to a minute hyaline vesicle, evidently by the escape and diffusion 

 of cell sap through the surrounding cytoplasm, which simultaneously 

 closes around and almost touches the centrosome. The centrosome 

 and astral rays, however, undergo little modification. 



The two pronuclei now come into contact; a slight vacuole is 

 left behind the egg nucleus at the animal pole in this final movement, 

 a fact which indicates that the movement takes place relatively 

 quickly, and before sufficient time has elapsed for the less fluid part 

 of the cytoplasm to come into a state of equilibrium behind the 

 moving pronucleus. 



This process occurs from about fifty until fifty-five minutes after 

 the first contact of the spermatozoon with the egg. If the fifteen 

 minutes which elapse before the spermatozoon penetrates through 

 the zona radiata into the cytoplasm be deducted, the entire time 

 occupied by the process of fertilization in Phascolosofua is forty 

 minutes, whereas in Toxopneustes , for example, it takes place, ac- 

 cording to Wilson (1895), in only eight minutes. This difference 

 may be due in part to the disparity in size between the eggs of 

 the two animals. 



The two pronuclei, at the time of their final coming together, 

 are almost identical in size. The asters, however, differ somewhat 

 in prominence. That which accompanies the sperm nucleus (Fig. 19) 

 is composed of long fibres, which extend in somewhat parabolic 

 curves around the pronucleus through the central protoplasmic field 

 of the cytoplasm, and even beyond it into the peripheral yolk-filled 

 region; the aster of the egg nucleus, though more prominent than 

 before the nuclei came into contact, occupies only a small region 



