156 EDOUARD A'AN BENEDEN. 



kern) which is the head of a spermatozoon. It is the result 

 of the conjugation of two nuclei. 



It is plain that there is in certain respects a remarkable 

 agreement between Hertwig's observations on Tozopneustes 

 and my researches on the rabbit. We have both ascertained 

 (1.) that there appears near the surface of the e^^ a clear space, 

 which I have called, in order not to prejudge in any way its 

 significance, the peripheral pronucleus. This peripheral 

 nucleus I have regarded, as at least in part, formed of 

 spermatic substance. (2.) This superficial nucleus penetrates 

 into the yolk and goes to meet another transparent body, of 

 which the character and significance are different from those 

 of the peripheral nucleus ; and which I have called the 

 central pronucleus in contradistinction to the peripheral pro- 

 nucleus. Hertwig calls it nucleus of the egg, in opposition 

 to his spermatic nucleus. 



(3.) We have both ascertained that the nucleus of the first 

 cleavage-sphere, called by me first embryonic nucleus, by 

 Hertwig cleavage-nucleus , is developed at the expense of the 

 two nuclear elements after they have joined and become united 

 in the centre of the yolk. We have both supposed that the 

 formation of the first embryonic nucleus results from the 

 union of a male element and a female element; and although 

 I have not applied the term conjugation to the fact which 

 essentially characterises the formation of the first nucleus, 

 still the idea was not the less present in my mind, at least 

 in a hypothetical form. 



The facts as to which we are in complete disagreement are 

 two in number. (1.) According to Hertwig, the germinal spot 

 does not disappear — it becomes the nucleus of the egg {Eikern), 

 According to my observations there exists no genetic bond 

 between the central pronucleus {nucleus of the egg of Hert- 

 wig) and the germinal vesicle or any of its parts ; the central 

 pronucleus which appears after fecundation is an element of 

 new formation. (2.) According to Hertwig, the peripheral nu- 

 cleus — the Spermahern — is the head of a spermatozoon, and 

 the transparent material surrounding it is protoplasm with- 

 out granulations. In my opinion the clear space which 

 appears in the cortical layer of the egg is a nuclear body 

 (the peripheral pronucleus) ; the refractive corpuscles which 

 appear on the spot are nucleolar elements. I propose in the 

 following pages to criticise the opinions expressed by Hert- 

 wig, and to make known the observations which I have had 

 the opportunity of making on the germinal vesicle of the 

 egg of an echinoderm belonging to the order Asterida ; 

 Asteracanthion rubens. 



