MUSEUM OF COMPARATIVE ZOOLOGY. 417 



which its membrane is very thick and resistant, — e. g. Osseous and 

 Elasmobranch Fishes, Birds, etc., — this may be incapable of complete 

 resorption, and be extruded bodily from the egg. lu the case of most 

 ova it is completely absorbed, though at a subsequent period it may be 

 extruded from the egg as the Richtungskorper. In all cases the contents 

 of the germinal vesicle remain in the ovum." 



In a paper on the germiuative vesicle and first embryonic nucleus 

 Ed. van Beneden (76«, pp. 38-76, and 76^, pp. 153-178) gives a 

 very minute account of the disappearance of the vesicle in Asteracan- 

 thion rubens. Much of this paper is taken up with a comparison of 

 his results in this case and in that of the rabbit, but more especially 

 with a comparison of his own results and those reached by Hertwig in 

 the study of Toxopneustes. We learn here ('76", p. 40) for the first 

 time definitely, that in the rabbit the nucleoplasm with the pseudo- 

 nucleoli forms the second of the two polar globules. He expresses here 

 more positively his conviction that the substance of the central pro- 

 nucleus is absorbed in an endosmotic way by the peripheral pronucleus. 



The vitellus of the Asteracanthion egg is composed of a clearer, less 

 granular cortical layer with radiated striations, and a central mass 

 which occupies two thirds the diameter of the egg. In the germinal 

 vesicle are to be distinguished the parts already described in the case of 

 mammals : a nuclear membrane enclosing a transparent and perfectly 

 homogeneous liquid ; a germinative spot formed of a very refringent 

 and brilliant substance enclosing a variable number of clear vacuoles ; 

 a reticulum of a finely granular substance (nucleoplasma) starting out 

 from the germinative spot as a centre and embracing in its substance 

 the pseudo-nucleoli. The latter vary in size and in number (from 8 

 to 15), and may be spread through the whole vesicle, but usually are 

 situated in the vicinity of the true nucleolus, from which they differ in 

 being much less refractive. 



Of nuclei in general Van Beneden holds, that the young nucleus is 

 formed of homogeneous matter, essence nucUaire. When it enlarges, the 

 nuclear essence becomes united with a substance (sue nucleaire) taken 

 from the protoplasm of the young cell. The substance nucleaire which 

 results from this union constitutes the body of the nucleus. The mem- 

 brane of the definite nucleus and the nucleoli are unmodified remnants 

 lof the primitive young nucleus ; they are formed exclusively of nuclear 

 \fissence. When a nucleus is about to divide, the nucleoli and the nuclear 

 jmembrane dissolve in the nuclear substance ; for this reason the contour 

 of the nucleus becomes very indistinct and the nucleoli disappear. After 



VOL, VI. — NO. 12. 27 



