MUSEUM OF COMPARATIVE ZOOLOGY. 473 



confirm his opinion that the essence of fecundation consists in a total or 

 partial renewal of the nucleus of the egg cell. 



Hensen (75, p. 238, Taf. VIIL Figs. 5-8) never saw " ein Saraen- 

 fadchen in den Dotter hinein kriechen," but has often seen these cor- 

 puscles imbedded, either entirely or the head only, in the yolk in the case 

 of the guinea-pig and the rabbit, and draws the general conclusion 

 (p. 241) that in the case of these animals more than one spermatozoon 

 can penetrate the yolk, where, under definite formal changes of the 

 head, it is dissolved, and that in this manner the fecundation of the egg 

 is accomplished, 



Ed. van Beneden ('75, pp. 693 - 695) was never able to observe the 

 penetration of a spermatozoon into the vitellus of the rabbit's egg ; but 

 from often finding spermatozoa very closely adherent to the siirface of 

 the yolk, he ventures to express the belief that " fecundation consists 

 essentially in the fusion of the spermatic substance with the superficial 

 layer of the vitelline globe." His account of the formation and union of 

 the pronuclei is given on pages 412 to 414. 



The penetration of spermatozoa into the egg, which Robin maintains 

 ('75, p. 21), does not imply a penetration into the yolk substance. The 

 ultimate molecular union of the substance of a large number of those 

 which penetrate the membrane and are liquefied, is evidently only an 

 inference from a supposed diminution of those still found in the peri- 

 vitelline fluid at later stages (see Robin '62, p. 87). " The retraction of 

 the yolk, the changes which supervene in its granules, the formation 

 of polar globules, are partial phenomena which occur with eggs whether 

 fecundated or not ; but the production of the vitelline nucleus only takes 

 place in ovules into which spermatozoa have penetrated, i. e. [in ovules] 

 to the vitellus of which male substance has been united." ('75, p. 86.) 

 Notwithstanding the accuracy of the greater part of this statement, it 

 does not follow that the author understood the true origin of the nucleus 

 of the first segmentation sphere, — 'his "noyau vitellin." In fact, it has 

 in his opinion an origin entirely independent of the germinative vesicle, 

 at the centre of the yolk, by a molecular association of " principes imme- 

 diats " of the vitellus. It is with the appearance of this nucleus that 

 the ovule takes on the characters of a new being, and ceases to be an 

 anatomical element of the adult animal which produces it. 



Thus, of all the parts which compose the ovule before maturity, the 

 vitellus, he believes, is the only one which serves for the production of a 

 new being. 



I shall not reproduce the second part of Van Beneden's ('76", pp. 76 - 



