MUSEUM OF COMPARATIVE ZOOLOGY. 485 



male pronuclei with the female pronucleus. A nucleus may be resolved 

 at once into a tetraster, — four asters united to each other. A vitellus 

 which has received two spermatozoa was never seen to develop normally, 

 but always produced double the normal number of spheres. 



Analogous phenomena (the penetration of numerous spermatozoa) are 

 observable with eggs fecundated at maturity, if they have come from 

 diseased animals. The bodies of the spermatozoa in this case remain 

 intact within thd vitellus, although surrounded with faintly expressed 

 radial striations. As their bodies are never found intact except in 

 these abnormal cases, Fol concludes that the male centre is produced 

 by the fusion of the " body " with a little vitelline protoplasm. The 

 mutual repulsion of the male centres he considers to be a corollary of 

 their attraction for the female centre, just as the mutual repulsion of 

 the poles of an amphiaster is a corollary of the attraction they exert 

 on the surrounding protoplasm. 



When the spermatozoa of Psammechinus miliaris come in contact 

 with the egg, their heads are, according to Giard ('77"), applied to the 

 whole periphery of the membrane (vitelline f), and they impart to the 

 sphere a very rapid gyratory motion. The vitelline membrane, hitherto 

 very near the surface of the yolk, gradually separates from it, and con- 

 sequently the second "cumulus" (see p. 444), whose summit adheres to 

 the membrane, is drawn out into a cone connecting membrane and 

 vitellus. As no spermatozoon is seen to penetrate into the vast clear 

 space which now intervenes between membrane and yolk, the author 

 believes that this cumulus serves for the passage of a spermatozoon ; 

 either that its summit corresponds to a pore in the membrane, or, as is 

 more likely, that the act of fecundation consists essentially in the diffu- 

 sion of male protoplasm through the membrane at the point where 

 the latter is in direct contact with the female protoplasm, i. e. at the 

 summit of the cumulus. This connecting cone soon detaches itself from 

 the membrane, and re-enters the vitelline mass. The male pronucleus 

 which results is not said to induce a stellate figure. The nucleolus of 

 the male pronucleus Giard thinks cannot be the unmodified head of the 

 spermatozoon. He is inclined to believe that the gyratory motion im- 

 parted by the spermatozoa facilitates the advance of the pronuclei 

 toward the centre of the yolk, since eggs for some time stationary are 

 developed irregularly. How this rotation facilitates the migration is 

 not stated by the author. 



In Fol's ('77^") illustrated paper on the " Commencement of Heno- 

 geny," etc., it is stated that the gliding of the body of the spermatozoon 



