EMBRYOLOGY OF CLEPSINE. 245 



the spindle-fibers, and in the centre of each is found at the 

 completion of the first cleavage, a cluster of four to six refrac- 

 tive corpuscles (nucleoli). 



15. That the bodies called pronuclei are composed of nuclear 

 substance is attested by their origin, general appearance, entire 

 history, and by the characteristic lead-grey, or blackish tinge 

 imparted to them by osmic acid. 



16. As soon as the cleavage-amphiaster is formed, the two 

 poles begin to move in opposite directions as if repelling each 

 other, and during this recession, which reaches its maximum in 

 the moment of cleavage, they ])ass from the round to tlie bi- 

 convex, and lastly to the meniscus form ; and at the comj)k'tion 

 of the cleavage they begin to approach each other again, passing 

 through these forms in inverse order. 



Their recession proceeds as if the force driving them asunder 

 was apjjlied to their inner face. 



General Considerations 

 [Relative to the phenomena above described). 

 a. Germinal vesicle (" vesicula germinativa," Puikinje). — 

 What is the destiny of the germinal vesicle? Does its history 

 as a nuclear element end with the maturity of the egg ? or does 

 it persist in whole or in part, and supplemented by the male 

 element, become the progenitor of the subsequent generations 

 of nuclei? Opinions have been, and still are divided. For 

 twenty-five years after the discovery of this body by Purkinje 

 (1825), it was almost universally believed that it became mor- 

 l)hologically extinct soon after the maturity of the egg. Within 

 the last twenty -five years this opinion has been contradicted, in 

 a more or less positive manner, by a considerable number of 

 very eminent biologists, among whom we may mention Joh. 

 Midler ('t?^% 1852), Leydig (^V, tot, 1854), Gegenbaur (-lif, 

 1854), Leuckart ('gV> 1«5S, f^), Keferstem (vTlSeS), Hac- 

 kel ( ff, 1869), van Beneden (if, 1870), Kowalevsky (If, 1871), 

 Frey \^%%, 1873), and v. Baer (^^^, 1876). On the other hand 

 many distinguished naturalists;, among whom are found some of 

 those just mentioned, have maintained the non-persistence of 

 the germinal vesicle. Of these we give the following : Baer 

 {-i^, 1846), Reichert {-^U, 1846), Leuckart (^f-f, 1853), 

 W'eismann (>f\ 1863), Knpfier (-^VV, 1870), Kleinenberg {%Ar, 

 1872), Oellacher (xi'^^, 1872), Auerbach {.J,^, 1874), Eobin 

 (VVS 1875), Flemming (-fV^, 1875), Gotte {^, 1875), Balfour 

 (Monogr. Develop. Elasmobranch Fishes, 1876), Kollikcr (-^, 

 1S76), and Bischofi" {\%, 1 877). To these may be added Iliickfl 

 (62) and van Beneden (16), who have both abandoned the behef 

 in the persistence of the germinal vesicle. 



