MtscellaneoHn. 353 



the knowlt'dijo of these j)rimordial pheuomena. M. O. Hertwig has 

 shown, in his tine memoir on the first development of the Kcliini, 

 that the Hpermatozoid ])enet rates into the ovum, and enters into tho 

 comjmsition of the niuleusof the fecundated ovum. I have repealed 

 M. Hertwifj^ observations and can warrant their correctness, ex- 

 cepting some details which will appear from my own description. 

 The body of the spermatozoid, when it has entered the vitellus, 

 appears to amalgamate with the vitelline protoplasm to form a clear 

 spot, which becomes tho centre of a system of radiating strioc. For 

 this 8|K)t I adopt tlie term promu-lms, proposed by M. K. van Bene- 

 den ; and I shall call it the mah pronucleus. This male pronucleus 

 traverses the vitellus to mingle intimately with a female pronucleus, 

 which is situated at the moment of fecundation in the part of tho 

 vit-elluB opjKisite to that through which the spermatozoid penetrates. 

 Derbes and M. (). Hertwig regard this female pronucleus as iden- 

 tical with the I'urkinjean spot of the ovule before its maturity. I 

 reserve my opinion upon this point, which I have been unable to 

 elucidate. From the fusion of these two pronuclei results the nucleus 

 of the fecundated ovum, which is afterwards segmented in the 

 manner described by me in a pre%aous note. 



In tracing the development of the Echinus, one is struck by the 

 complete absence of any polar corpuscle. This evidently constitutes 

 a very exceptional case in the animal kingdom. In the immense 

 majority of cases the ripe ovule possesses a large germinal vesicle, 

 which only disappears at the moment of fecundation (>Sa(/itta), or a 

 little later {Merotrachca, Aster las, &c.). This germinal vesicle is 

 immediately replaced by a system of filaments arranged in a double 

 star, absolutely as in a cell which prepares to divide, only this 

 system is situated cjuite close to the surface of the ovum. The more 

 peripheral star then issues from the vitellus to constitute a polar 

 corpuscle, which may divide after its escape : most frequently it 

 remains entire, and the star remaining in the interior of the vitellus 

 divides into two stars, one of which issues to constitute the second 

 polar corpuscle. The substance expelled in this manner repre- 

 sents the greater part of the germinal vesicle enveloped by a little 

 vitelline protoplasm. The opinion of Gillacher as to the origin of 

 these corpuscles in the Trout finds a brilliant confirmation in these 

 facts. The last star that remains in the vitellus collects to form a 

 pronucleus. 



At this moment I have observed in Sar/itta and various Gastero- 

 poda a clear spot which forms at the opposite pole of the vitellus. 

 This spot is surrounded, in Sayitio, by a star of protoplasmic fila- 

 jments. It moves in the direction of the spot where the other 

 pronucleus is placed. During this movement of translation we see 

 very clearly, in Sagittu, that the centre of the star occurs in front 

 of the dear spot, and that the latter is pjissively drawn along. On 

 its arrival close to the other pronucleus, hitherto motionless, this 

 star moves more rapidly, the pronucleus is drawn towards the clear 

 spot, and these two elements fuse together to form the nucleus of 



