HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 25 



discoidal aggregation of protoplasm takes place, and towards which after the forma- 

 tion of the second polar globule the female pronucleus gravitates." The same 

 leading part has been demonstrated for the male pronucleus in Petro- 

 myzon by Bohm. 



8. The Female Pronucleus. — Ten minutes after fertilization the first 

 polar globule rises plainly above the surface of the blastodise, but the 

 division of the spindle and the separation of the globule are not, yet 

 complete (Fig. 27. PI. XXIII.). Within five minutes the division is ended 

 and the second spindle formed. The mode of formation of the second 

 spindle has entirely escaped us. The rapidity with which the operation 

 is performed is probably partially accounted for by the fact, made known 

 through the investigations of Boveri (No. 13) and Platner, 21 that the rest- 

 ing phases of the nucleus are skipped, the inner nuclear plate of the first 

 spindle passing directly into the second spindle. 



The division of the second spindle is thus a reduction of the nuclear 

 elements, but not necessarily a reduction in the sense of Weismann. 22 

 Platner calls attention to the remarkable fact, that this skipping of the 

 resting phases occurs in only two cases: first, in the formation of the 

 second polar spindle; and secondly, in the formation of the final spindle in 

 the production of the spermatozoa. As the spermatosomes are equiv- 

 alents, Platner suggests that the egg and the polar globules must be 

 regarded in the same light. 



Twenty minutes after fertilization (Figs. 13-20, PI. XXII.), the female 

 pronucleus is well advanced, and already en route to join the male pro- 

 nucleus. The chromatic elements (ovomerites of Bohm) are about the 

 same in number, size, and general appearance as those of the male 

 pronucleus; the achromatic periplast, to use Vejdovsky's term, is much 

 smaller than in the male pronucleus, and frequently eludes detection. 

 Some of our drawings indicate a very feeble radiation around this pro- 

 nucleus, but the existence of astral rays comparable to those seen in 

 the male pronucleus cannot be positively affirmed. The identification 

 of the female pronucleus is rendered possible by its relation to the polar 

 globules, as in Figs. 13-20. When the female pronucleus gets near 

 the male, as shown in Figs. 15-17, the clear periplasts appear to be 



21 G. Platner Ueber die Bedeutung der Richtungskorperclien. Biolog. Centralhlatt, VII., No 23, 

 Feb. 1, 1889, p. 718. 



22 August Weismann Ueber die Zabl der Richtungskorper nnd iiber ihre Bedeutung fiir die Ver- 

 crbung, Jena. 1887- 



4 



