250 INTERNAL FACTORS IV. i 



Garbowski, Driesch), the median plane of the Pluteus. In this 

 case there is practically no distinction between ' penetration ' and 

 'copulation' parts of the sperm-track, and, the centrosome divid- 

 ing perpendicularly to this path, the point of entrance naturally 

 lies in, or nearly in, the first furrow. The other alternative, as 

 Wilson has pointed out, is to suppose that the definitive polarity 

 of the egg existed before fertilization, but exerted no influence 

 upon the egg-nucleus, although able subsequently to compel the 

 fertilization nucleus to take up a position in its axis. 1 There 

 are other Echinoderms, however (Asterias, Strongylocentrotus), in 

 which the original and the definitive egg-axes coincide. 



Another very interesting case is Cynthia (Fig. 142). In the 

 immature egg of this Ascidian the cytoplasm is concentrically 

 arranged, since the yellow pigmented protoplasm forms a peri- 

 pheral investment for the central grey yolk : the egg-axis is de- 

 termined only by the position of the germinal vesicle. Upon the 

 entrance of the spermatozoon the yellow substance together with 

 the clear plasma of the germinal vesicle, which has in the mean- 

 time broken down, streams towards the vegetative pole and becomes 

 radially arrayed about the axis. By a further streaming movement 

 these two substances are carried up with the male pronucleus 

 or pronuclei in cases of dispermy to the equator, so marking 

 the future posterior side of the now bilateral egg. These changes 

 are evidently an effect of fertilization : it cannot, however, be 

 asserted that the point of entrance of the sperm determines the 

 posterior end, since it may enter on the opposite side and get 

 carried across. 



There are a few other instances in which an alteration of the 

 egg-structure has been noticed to follow on fertilization. In 

 Ctenophora the peripheral layer of granular cytoplasm becomes 

 aggregated at the animal pole (Agassiz) ; in Polyclads (Lang) and 

 Cirripedes (Groom) the egg becomes telolecithal ; and so in Pbysa 



1 In Diplogaster longicauda also it appears to be the point of union 

 of the two pronuclei which determines the definitive egg-axis and 

 orientation of the embryo. The polar bodies are always extruded and 

 the female pronucleus formed at that end of the ellipsoid egg which is 

 turned towards the ovary, while the sperm enters at the opposite end. 

 The pronuclei may meet and unite at either end ; but it is at the end 

 where their union takes place that the smaller of the first two blasto- 

 meres is found and, later, the hinder end of the embryo (H. E. Ziegler, 

 Zeitschr. wiss. Zool. Ix, 1890). 



