GERMINAL VESICLE AND FIRST EMBRYONIC NUCLEUS. 163 



rounding plasma." The question as to what becomes of 

 the membrane has not as yet been resolved; but what 

 is certain, according to Kleinenberg, is that every trace 

 of the germinal vesicle has already disappeared long before 

 the moment when fecundation takes place. 



Thanks to the observations of Oellacher and Kleinenberg, 

 the disappearance of the germinal vesicle was directly 

 demonstrated ; from that time it was no longer possible to 

 maintain the persistence of the germinal vesicle in all 

 animals, without denying the facts observed. The question 

 then entered into a new phase. Two observers, who must 

 certainly be reckoned amongst the most eminent of the age, 

 had just established the mode i?i which the germinal vesicle 

 disappears. From that time only two alternatives were 

 further possible ; either it must be admitted that the 

 germinal vesicle does not play the same part in all animals ; 

 that it disappears in some, and that it persists and under- 

 goes division in others ; or else it must be allowed that all 

 the observations made by Miiller, Leydig, Gegenbauer, 

 Leuckart, Pagenstecher, Mecznikow, KoUiker, Haeckel, and 

 myself were erroneous, or, at any rate, that the conclusions 

 drawn from the facts observed were but little in conformity 

 with the principles of logic. I believe that the latter of these 

 hypotheses is of the two more probable ; the opinion which 

 affirmed the permanence of the germinal vesicle rested, in 

 fact, on negative grounds. It was affirmed that this element 

 does not disappear, because no egg had ever been found 

 which was entirely devoid of all central nucleus; but it does 

 not strictly follow that because no egg had been found 

 entirely deprived of a central nucleus, that therefore the 

 germinal vesicle persists. The doubts which the researches 

 of Oellacher and Kleinenberg had aroused in my mind led 

 me to make fresh researches. In order to make fresh obser- 

 vations on this point it was of importance to choose eggs in 

 which the yolk possessed in the greatest possible degree the 

 properties of transparency and homogeneity, and it was 

 requisite, moreover, that they should be distinguished by the 

 dimensions of the germinal vesicle and the (germinal) spot of 

 Wagner. The ova of the Echinodermata, and in particular 

 those of the Asteracatithio/i rtibens realise these conditions in 

 the highest degree. At the end of April, 1874, I betook 

 myself to Ostend with the object of carrying out the artificial 

 fecundation of these ova. 



It is not very long ago that we were still in ignorance as 

 to whether the sea starfish are of different sexes. Tiedeniann 

 declares that that he has never found the male organs of these 



