396 F. M. BALFOUR. 



be found, chiefly in the deeper layers of the ijermiiial epitlieh'um, 

 larger ova difl'ering considerably from the primitive ova. They form 

 the permanent ova (PI. XVII, liir. 3, o.). Their average diameter 

 is 0"U1< mm., compared with 0'0-3 ;nm., the diameter of original 

 primitive ova. The protoplasm of which they are composed is 

 granular, but at first a membrane can hardly be distinguished 

 around them; their nucleus is relatively large, 0"02 — 0'027 mm. 

 in diameter. It presents the characters ascribed by Eimer,^ 

 and many other recent authors,^ to typical nuclei (vide PI. 

 XVII, fig. 3, and PI. XVIII, figs. 13, 14, 15, 16, 17, 

 18). It is bounded by a distinct membrane, within which is a 

 more or less central nucleolus from which a number of radial 

 fibres which stain very deej)ly pass to the surface; here they 

 form immediately internal to the membrane a network with 

 granules at the nodal points. In some instances the regularity 

 of the arrangement of these fibres is very great, in other instances 

 two central nucleoli are present, in which case the regularity is 

 considerably interfered with. The points in which the youngest 

 permanent ova ditt'er from the primitive may be summed up as 

 follows : — 



(1) The permanent ova are larger, the smallest of them being 

 larger than the average primitive ova in the proportion of four 

 to three. (2) They have less protoi)lasm as compared to the 

 size of the nucleus. (3) Their protoplasm is granular instead 

 of being clear. (4) Their nucleus is clear with exception of a 

 network of fibres instead of being granular as in the primitive 

 ova. It thus appears that the primitive ova and permanent ova 

 are very difl'erent in constitution, though genetically related in a 

 way to be directly narrated. 



The formation of permanent ova is at its height in embryo of 

 about 7 centimetres or slightly larger. The nests at this stage 

 are for the most part of a very considerable size and contain 

 a large number of nuclei, which have probably, as before in- 

 sisted, originated from a division of the smaller number of 

 nuclei present in the nests at an earlier stage. Figs. 14 — 18 

 are representations of nests at this period. The diameter of the 

 nuclei is, on the whole, slightly greater than at an earlier stage. 

 A series of measurements gave the following results : — 



OOlf) mm. 

 OOK) mm. 

 0018 mm. 

 002 mm. 

 0-02 mm. 



lioih varieties of modified nuclei are common enough, though 



' ' Arcliiv f. micr. Aiiat.,' vol. xiv. 



' Fide especially Klein, in tlu; last number of this Journal. 



