8 Hjalmar Théel, 



never succeeded in finding any polar cells, these having certainly been 

 torn off by constriction from the egg inside the ovarial tubes, as is known 

 to be the case in other Echinoids, and then removed. Fewkes and 

 Nachtkieb also did not notice any polar cells in the two Clypeastroids, 

 Echinai-achnius and Mellita. However, I did not spend much time in 

 searching for them, considering it to be beyond the limits of this report. 



As soon as the spermatozoa, which have a length of 0,06 mm., 

 PL I, fig. 20^ have penetrated the mucilaginous investment and come 

 into contact with the egg itself, the latter, which hitherto has passed a 

 somewhat latent existence, begins to undergo a series of very interesting 

 processes which are well known in other Echinoderms and therefore 

 need not be repeated here in detail. 



It is especially interesting to observe the operations of the sper- 

 matozoa. If a single egg be placed in a humid chamber arranged on 

 an object-glass and a few spermatozoa be admitted at some distance 

 from the egg, a very exciting display takes place under the eyes of the 

 observer. The spermatozoa show a high degree of vivacity and run 

 searching in all directions as if they had a sense of the remote ovum. 

 One has a distinct impression that here ihere must be either two attrac- 

 tive forces, the egg being the more powerful, or a conscious activity 

 on the part of the spermatozoa. 



With regard to the changes which take place in the impregnated 

 egg, I only refer to the following. At the place where the first sper- 

 matozoon has penetrated the mucilaginous investment, a very thin plas- 

 matic vitelline membrane rises and separates from the egg, Fl. I, fig. 1, 

 beginning at the place of contact and extending eventually round the 

 yolk, the consequence of which is that the yolk shortly appears as if 

 situated in the centre of a large, spherical, sharply defined sac, the inter- 

 mediate space being filled up hy a homogeneous transparent plasma, PL 

 /, fig. 2. 



It is not yet decided whether the above membrane is present be- 

 fore the impregnation, or arises just at the moment of contact between 

 the spermatozoa and the egg. For my own part, I am inclined to think 

 that a plasma-membrane is always differentiated before the act of ferti- 

 lization. At the place where it first begins to rise, the outline of the 

 membrane does not present itself as a smooth even line but runs in un- 

 dulations, and passes distinctly, as it appears to me, into the larger re- 

 maining part of the membrane, which is still closely pressed against the 

 yolk. It is only at a later moment that it becomes expanded and assumes 



