34 SEGMENTATION AND FORMATION OP THE LAYERS. 



Finally, this ovum possessed the peculiarity of presenting in 

 surface views (PI. I, fig. 3) a number of opaque patches. 

 These in section are seen to be due to a number of peripher- 

 ally placed areas in which the protoplasmic reticulum was 

 dense as it is around the female nucleus. The protoplasmic 

 reticulum of these denser areas was arranged in a radiating 

 manner around a central point ; it presented no deeply-staining 

 masses of chromatin. 



b. Two Ova in which the second polar body was 

 being formed. — In both of these the nucleus of the ovum 

 had already divided into the definite female pronucleus and the 

 nucleus of the second polar body, which in each case was 

 attached by a wide base to the ovum. In one, however, this 

 division has only just occurred, and the female pronucleus was 

 in the form of some small deeply-staining masses placed close 

 to the surface of the egg ; the denser protoplasmic reticulum 

 of the animal pole around them not being apparently modified. 

 The male pronucleus presented the same features as in the 

 last described ovum. 



In the other ovum the female pronucleus (PI. Ill, fig. 1) 

 was in a very different condition to the above. The chro- 

 matin masses had acquired a definite relation to the pro- 

 toplasmic reticulum, and the whole structure resembled 

 in all its essential features the chambered nucleus of the 

 fertilised ovum (see above, p. 31). Its greatest diameter 

 was "029 mm. At the opposite side of the ovum and not 

 quite in the same plane (though for the sake of convenience 

 the two structures are combined in one figure), there was a 

 large ('025 x "016 mm.) reticulated structure, which I take 

 to be the male pronucleus (PI. Ill, fig. 1). This male pro- 

 nucleus was much nearer the centre of the egg than those 

 previously described, as though it were in the act of moving to 

 the female nucleus. The network in this nucleus was of vary- 

 ing degrees of fineness, and was more deeply stained in some 

 parts than in others ; the main strands were obviously continuous 

 with the surrounding membrane, which in its turn was obviously 

 continuous with th§ very loose reticulum outside. 



