MUSEUM OF COMPARATIVE ZOOLOGY. 219 



recently been learned concerning the relation of egg and spermatozoon. 

 In one other case, alluded to above, a perfectly homogeneous oval vacuole 

 (Fig. 70^, ^) was observed near the surface of the yolk at the equator. 

 The female pronucleus of the same egg (Fig. 70^, a) was of nearly the 

 same size, but contained a single large nucleolus. In all cases the vacu- 

 ole seemed filled with a substance of less density than the surrounding 

 portfons of the vitellus. 



As already indicated, the male pronucleus in its more advanced stage 

 is so nearly similar to the female pronucleus in all morphological 

 points, that it would be mere repetition to describe it in detail. Not 

 only the size of the two at any given time, but also the form, the char- 

 acteristic behavior with different reagents, the appearance of the nucleoli, 

 even the number of the latter, are subject to such unimportant differences 

 that it would be quite impossible for one, however familiar with them, to 

 say which was the male and which the female element, were it not for 

 the positions which they occupied relative to each other and the remain- 

 ing parts of the vitelline sphere. 



Inasmuch as there exists a definite relation between the size of these 

 pronuclei and their distance apart, — which may be expressed by say- 

 ing, the larger the pronuclei, the nearer they will be to each other, — 

 it might be justly inferred, even without the corroborative evidence of 

 direct observation in the eggs of many other invertebrates, that migra- 

 tion of one or the other of them takes place. From what has already 

 been said of the migration of the female pronucleus, it may at once be 

 inferred that this approximation takes place principally, if not exclusively, 

 by a change in the position of the male pronucleus. 



When within a short distance of each other, the two (Fig. 68) are 

 often seen with their more pointed ends directed toward the centre of a 

 stellate figure. In case acetic acid has been used, the form becomes 

 much altered, and it is no longer possible to observe so clearly this con- 

 dition ; but even here their mutual relationship to the stellate^figure may 

 often be very easily demonstrated (Figs. 57, 59). 



Notwithstandmg that in many cases this position may be shown, even 

 in an advanced state of the pronuclei, in other cases the female pronu- 

 cleus appears to be more or less coincident with the central portion of 

 the stellate figure (Fig. 72), although it probably never happens that 

 the ce7itre.s of the two structures exactly coincide. In reality this aster 

 often becomes quite invisible before the close approximation of the two 

 pronuclei has been effected. 



There may then be left for a time an irregular area in its place, where 



