MUSEUM OF COMPARATIVE ZOOLOGY. 211 



the thickenings first unite to form a homogeneous mass, in which new 

 bodies arise as nucleoli. A few facts seem to point to the latter as the 

 more probable explanation. I have rarely seen a case, it is true, in 

 which this vesicular structure was entirely homogeneous (Fig. 70*) ; yet 

 the small number of the enclosed bodies in the earlier stages, as well as 

 their noticeable inequality of size, are facts not easily reconciled with 

 the notion of a direct conversion of the thickenings into nucleoli. The 

 constant increase in the number of these corpuscles indirectly favors 

 the idea that all are new productions within the homogeneous nuclear 

 mass. 



For reasons which are already familiar to those acquainted with 

 the phenomena of this stage of development, and which will be consid- 

 ered later in the present paper, this vesicular structure will be called the 

 female pronucleus (fpn), and the contained bodies female pronucleoli 

 (fpnl). The opacity of the yolk prevents observation on the living egg 

 of the changes which accompany the origin of this pronucleus ; so that the 

 conclusions are necessarily of the nature of inferences, capable, however, of 

 some degree of control. As previously stated, the earliest stage showing 

 unequivocally the existence of this pronucleus is one in which pronu- 

 cleoli are already present. But in some eggs, as in that represented 

 in Fig. 63 (compare Fig. 70**), a confused mass occupies the place of the 

 lateral zone of thickenings, and may fairly be taken, I believe, as the in- 

 cipient stage of this pronucleus, which does not as yet show any distinct 

 traces of nucleoli. Treatment with osmic acid is generally much more 

 sel-viceable for the discovery of this pronucleus than that with acetic 

 acid. Possibly this may explain why some of the eggs subjected to the 

 latter do not exhibit any distinct evidence of the lateral zone, or the 

 pronucleus (Figs. 66, 67), when, to judge from other features, we might 

 look with confidence for such structures. 



At a later stage (Figs. 57-60) this female pronucleus is found still 

 occupying a position near, but certainly not coincident with, the centre 

 of the stellate figure. Its outline after treatment with acetic acid is 

 quite distinct, and exhibits a double contour, which is usually more or 

 less wrinkled (Figs. 57, 59). After treatment with osmic acid, however, 

 the outline appears even, and there is no double contour to suggest 

 the existence of a distinct nuclear membrane. In both methods of treat- 

 ment followed by staining, the pronucleus is somewhat more deeply 

 colored than the surrounding protoplasm ; especially is this noticeable 

 in the osmic acid treatment. By either method it contains a number of 

 rounded bodies, varying from 2 //, to a minuteness bordering on the lim- 



