MUSEUM OF COMPARATIVE ZOOLOGY. 443 



ment, especially since a mutual approach of the vesicles is also to be 

 observed while they remain quite spherical. A third cause, he believes, 

 is to be sought in a contractile connecting substance stretched between 

 the two vesicles, in the form of a protoplasmic network, since it is ques- 

 tionable if the contractility of the yolk can effect a regular approach of 

 the vesicles (pp. 371, 379). Finally the latter are completely fused. 

 The germinative vesicle in the egg of the nematode is neither dissolved 

 nor otherwise destroyed. 



GiARD (77) gives a description of hyaline spheres which in Rhizo- 

 stoma make their appearance near the surface of the egg, and at its 

 maturity constitute a clear zone just underneath the vitelline mem- 

 brane. As previously (p. 332) stated, he ascribes to Butschli the 

 discovery that the polar globules are formed in many animals by a 

 process of cell division. With all these animals the excreted corpuscles 

 have the value of rudimentary cells of an atavistic signification,* and 

 cannot be properly called "corpuscules de rebut." The latter name is 

 only appropriate for non-cellular material rejected by the vitellus which 

 serves for the formation of accessory organs, the vitelline membrane, 

 for example. It is with the latter that the hyaline vesicles in Rhizo- 

 stoma are to be classed. 



The results reached by Giard ('77") in studies principallj'- on the eggs 

 of Psammechinus miliaris confirm in many points the observations of 

 Fol ; in others, his conclusions are different. I shall notice especially 

 their disagreements. The egg of this sea-urchin possesses a very delicate 

 vitelline membrane even before fecundation. A little before maturity the 

 germinative vesicle presents the reticulum characteristic of old nuclei. 

 The nucleolus embraces an irregular nucleolinus. The contents of the 

 vesicle become mingled in an amoeboid mass, attain the surface of the 

 yolk, and there are converted into a karyolytic figure. The aster di- 

 rected toward the centre of the egg very rapidly assumes the form of a 

 rounded nucleus, — the structure 0. Hertwig took to be the germinative 

 spot. It cannot be the " spot," for it always appears a little smaller than 

 the latter, and moreover one often encounters eggs in which this Wag- 

 nerian spot is no longer visible, and in which the female pronucleus does 

 not yet present a distinct nuclear aspect. On the other hand, it is inex- 

 act to say that there is no genetic connection between the two (Fol), since 

 the substance of the nucleolus, mingled with that of the germinative 

 vesicle, serves for the formation of the first amphiaster, which gives rise 

 to the female pronucleus. Giard describes the formation of two polar 



* See also Giard '76. 



