178 EDOUARD VAN BENEDEN. 



vesicle or one of its parts and the first nucleus of the 

 embryo. 



But whilst in the Stai-fish the collection of the nucleo- 

 plasma, the germinal spot and the membrane of the germinal 

 vesicle are dissolved in the nuclear fluid and secondarily in 

 the protoplasm of the yolk — in the mammalia these elements 

 are thrown off into the perivitelline liquid to form directive 

 bodies, and only the contents of the germinal vesicle remain 

 in the yolk. Since in the starfish directive bodies are elimi- 

 nated by the yolk, it is probable that in the Echinodermata, 

 as in Mammalia, these bodies are formed by the nucleoplastic 

 substance on the one hand, and by the nucleolar matter, 

 joined to the substance of the membrane, on the other hand. 

 It must be admitted that this is a pure hypothesis. But, 

 however this may be, it follows from my observations that 

 in the starfish as well as in the rabbit, there is no filiation 

 whatever between the germinal spot and the first embryonic 

 nucleus. 



II. There is a second point in the observations and views 

 of M. Hertwig, which appears to me irreconcilable with the 

 results of my researches on the rabbit. 



Is the clear spot which appears in the cortical layer of the 

 yolk protoplasm loithout granulations, and is the corpuscle which 

 is found there, and which Hertwig regards as the head of a sperma- 

 tozoon a cell-nucleus ? Or is the clear spot a nuclear body and the 

 corpuscle contained in it a micleolar element having no morpho- 

 logical connection with a spermatozoon ? 



The spermakern of Hertwig is the head of a spermatozoon 

 enclosed in a clear spot. This is composed of protoplasm 

 without granulations. My peripheral pronucleus, which is 

 certainly homologous with Hertwig's clear spot and with 

 the peripheral nuclei of Auerbach, Biitschli, and Strasburger 

 is, according to the terms employed in my preliminary commu- 

 nication, "a small, round homogeneous body, without granu- 

 lations ; it has, in fact, the appearance of a vacuole. But 

 when treated by osmic acid, the clear substance of the so- 

 called vacuole becomes darker and of a grey colour, while 

 the substance of the yolk is coloured brown." It is only later 

 when the pronucleus is already buried in the yolk, that 

 several very highly refractive corpuscles, which would be 

 taken for so many nucleoli, if seen in an ordinary nucleus, 

 appear in its interior. If then our observations agree so far, 

 that we have both seen the body which appears in the yolk, 

 near the surface of the eg^ with (according to Hertwig) one, 

 or (according to me) several refractive corpuscles, we differ 

 (a) as regards the moment of appearance of the nucleoli-form 



