256 FIELD. [Vol. XI. 



penetration of the Echinoderm spermatozoon into the q^^, I 

 have never seen the cytoplasm of the ^g^ send up a little pro- 

 jection to meet the incoming spermatozoon, as described by 

 Fol, and now incorporated into all the text-books. On the 

 contrary one sees a depression in the surface of the cytoplasm 

 caused by the mechanical pressure of the spermatozoon ; ex- 

 actly the same condition as when one presses a sharp pin into 

 a piece of rubber. The path made by the nucleus as it plows 

 through the cytoplasm is plainly visible (Fig. 57). 



The female pronucleus moves towards the advancing male 

 pronucleus in this way : at first spherical, it takes on a pear 

 shape, the narrower apex being directed towards the male pro- 

 nucleus. This anterior projecting process reminds one of a 

 very large, blunt pseudopodium of an amoeba ; and the pro- 

 nucleus travels by an almost amoeboid motion ; the shape of the 

 nucleus constantly changing, the projection being sent out and 

 the posterior portion pushing forward and filling it out. 



Tail. — The tail, a round flagellum about 0.211, 0.3 /u, in 

 diameter varies in length not only in the different groups, but 

 also in different individuals of the same species. In the 

 species studied the extremes are found among the Holo- 

 thurioidea ; in SticJwpus regalis it is very short, about 40.0 /*, 

 while in HolotJmria Polo it is about 90.0 \x. The other species 

 studied showed all intermediate lengths. 



As Pictet has already shown for the Echinoidea (18) the tail 

 is formed from the cytoplasm of the spermatid. Although I 

 can add but little to the results obtained by him, more than to 

 confirm them for representatives of all the classes of Echino- 

 derms, yet for the sake of more fullness I will go over 

 the subject here. Soon after the mitotic division of a sper- 

 matocyte into two spermatids, the cytoplasm of the spermatid 

 begins to form a bulging which increases into a large projection 

 like an enormous blunt pseudopodium (Fig. 40) ; the cytoplasm 

 continues to push or flow into this projection and it becomes 

 elongated and flask-shaped, the body of the flask consisting of 

 a large drop of cytoplasm, which is connected with the cyto- 

 plasm proper by a narrow neck (Figs. 42, 43). The continued 

 lengthening of the tail takes place with the elongation and 



