180 EDOUARD VAN BENEDEN. 



not become the nucleus of the first cleavage-sphere till it 

 has become united to another element having also the ap- 

 pearance of a cell-nucleus. It is for this reason that I have 

 spoken of the peripheral body as the peripheral pronucleus, 

 and of the element which is formed in the centre of the 

 yolk as the central pronucleus. 



If we grant what appears to me hardly to admit of doubt 

 that the small highly refractive bodies of my peripheral 

 pronucleus are homologous with the spermakern of Hertwig, 

 and that in the same way as there are nuclei with a single 

 nucleolus and nuclei with many nucleoli, so also there are 

 pronuclei with a single corpuscle, and others with many 

 corpuscles, then it seems to me that Hertwig's view of his 

 spermatic nucleus being the head of a spermatozoon has 

 little probability. In Mammalia the nucleolar elements of 

 the peripheral pronucleus have certainly not this significance. 



(1.) The pronucleus at the moment of its appearance is in 

 the rabbit without any granulations. Now this would be 

 inconceivable on Hertwig's hypothesis ; the formation of the 

 clear spot is, according to him the consequence of fecunda- 

 tion ; now, fecundation begins with the presence of the sperma- 

 tozoon. The formation of the spot must then always be 

 consecutive to the penetration of the spermatozoon. The 

 spot must accordingly be formed around the head of the 

 spermatozoon, for the head can never penetrate into a pre- 

 formed spot. We ought then never to see a peripheral pro- 

 nucleus without at least one nucleolar corpuscle. 



(2.) The pronucleus in Mammalia contains several granu- 

 lations. These granulations have neither the appearance nor 

 the dimensions of the heads of spermatozoa; they are 

 spherical or oval globules, the dimensions of which show 

 much variation j the smallest are almost points, and even the 

 largest have not half the size of the heads of spermatozoa. 



(3.) Auerbach, Biitschli, and Strasburger, observed before 

 Hertwig and before myself, in the Nematodes, the MoUusca, 

 and the Ascidians, sometimes one, sometimes several clear 

 elements appear near the surface of the Qg^. All three 

 describe these bodies as globules which are at first homo- 

 geneous, and are, at the moment they appear, quite without 

 granulations. According to the observations of Auerbach, 

 which on this point quite agree with all that I have observed 

 in Mammalia, corpuscles, three to six in number, which 

 Auerbach calls nucleoli, afterwards appear in the homo- 

 geneous liquid of these globules. There is not the slightest 

 analogy between these corpuscles and the Nematode sper- 

 matozoa. 



