456 BULLETIN OF THE 



from which an egg nucleus (in Hertwig's sense) arises. The stages of 

 this metamorphosis are not very completely known. Eggs taken between 

 the middle of October and the middle of November from animals ap- 

 proaching maturity exhibit the germinative vesicle, still sharply outlined 

 and already arrived at the periphery of the yolk. Those taken toward 

 the end of November and at the beginning of December, on the contrary, 

 show that the vesicle has already lost its germinative dot and its sharp 

 contour, and only its protoplasm lies in an irregular form at the periph- 

 ery. Within this mass of protoplasm are observable, in the fresh state 

 of the egg, " all sorts of nuclear structures," which are probably descend- 

 ants of the germinative dot. In many eggs, however, — and these the 

 largest in the ovary, — there was nothing to be seen of a germinative 

 vesicle or nuclear structures ; there was only a clear drop of protoplasm 

 at one point of the periphery. Already, on the 9th of December, the 

 eggs of a completely metamorphosed larva exhibited a new nucleus 

 (Eikern) in this clear mass of protoplasm or remnant of the germinative 

 vesicle. Calberla thinks, without having recorded any direct observa- 

 tions of such an act, that a part of the vesicle is eliminated as the polar 

 globule. The new nucleus then migrates toward the centre of the egg, 

 drawing after it a cord of protoplasm destitute of yolk granules. Thus 

 a month or more before the maturity of the egg one finds the following 

 complications of structure. The egg membrane is thickened and exhibits 

 a micropyle at its narrow end where the germinative vesicle approaches 

 the surface ; this he calls an outer micropyle, to distinguish it from the 

 entrance to a canal — " Spermagang " — formed directly underneath it in 

 the granular yolk by the centripetal migration of the egg nucleus and 

 the clear protoplasm it carries with it. The entrance to this latter canal 

 is the inner micropyle. Protoplasm which is destitute of granules en- 

 velops the granular yolk on all sides, and is thickened at this, the animal 

 pole, where it is continuous with the likewise clear protoplasm that fills 

 the " Spermagang." Within the enlarged deeper end of the latter the 

 egg nucleus lies surrounded on all sides by a stratum of this clear pro- 

 toplasm. 



Galeb (78«, pp. 363-366, PI. XXII. Figs. 1-4) maintains, on 

 much the same ground as the embryologists of ten and twenty years 

 ago, that the germinative vesicle persists, and (without any fibrous meta- 

 morphosis) undergoes a simple elongation, constriction, and ultimate di- 

 vision to form the unequal nuclei of the first pair of blastomeres.* He 

 seems to have taken no measures to insure himself against the possibil- 



* See also the review at p. 334. 



