GERMINAL VESICLE AND FIRST EMBRYONIC NUCLEUS. 181 



(4.) Refractive corpuscles, perfectly identical with those 

 which arise in the peripheral pronucleus were seen also in 

 the central pronucleus. 



(5.) Corpuscles which perfectly resemble those observed in 

 the peripheral pronucleus are found also in the completely 

 developed nuclei of the cleavage-spheres, and these corpuscles 

 are certainly nucleoli. 



The opinion expressed by Hertwig that the transparent 

 body which is formed near the surface of the egg is not a 

 nuclear element, but protoplasm without granulations, 

 appears to me untenable, so far as regards Mammalia, for the 

 following reasons : 



(1.) The transparent substance of the peripheral pro- 

 nucleus does not behave with re-agents like the protoplasm of 

 the yolk, but like the substance which forms the cleavage- -'^■•^'' 

 spheres. If the pronucleus be treated with a one-per-cent. 

 solution of osmic acid, it is not coloured brown like the pro- 

 toplasm of the yolk ; on the contrary, it appears on the brown 

 ground of the yolk as a clear well defined spot. The pro- 

 nucleus is faintly coloured pink by picrocarminate applied 

 after osmic acid. By hsematoxylin it is coloured violet-blue. 



(2.) When the peripheral pronucleus has become attached 

 to the central pronucleus, it presents after a time the same 

 appearance as the latter, enlarges, and its outlines become 

 more and more distinct. The nucleolar elements remain for 

 a certain time ; finally they disappear and the enlarged peri- 

 pheral nucleus, not one of the refractive corpuscles, becomes 

 the first embryonic nucleus. 



(3.) My view is in harmony with the opinion expressed by 

 Auerbach, Biitschli, and Strasburger, who have all through 

 regarded the transparent bodies formed at the periphery of 

 the egg as nuclear elements. 



From all the foregoing considerations I conclude that 

 Hertwig's view respecting the mode of formation, constitu- 

 tion, and morphological significance of the transparent bodv 

 which appears near the surface of the egg in Toxopneustes 

 lividus is very improbable. I regard this body, which is 

 homologous with the peripheral pronucleus of the rabbit, as 

 being a nuclear element, and I regard the corpuscle con- 

 tained in it which Hertwig calls spermatic jiucleus not as the 

 head of a spermatozoon, but on the contrary, as a nucleolar 

 element homologous with those which exist in large numbers 

 in the peripheral pronucleus of Mammalia, Nematodes and 

 Ascidians. 



I have found the number of these nucleolar elements to 

 be very variable in Mammalia. In the rabbit the number 



