4^4 The American Naturalist. [June, 
cell before maturation ; it is usually membranate, and has nu- 
merous fine granules of chromatine, microsomata, derived from 
the pronuclei ; in some cases the microsomata from the male 
pronucleus are distinguishable from those of female pronucleus. 
In the rabbit the nucleus when first formed has indistinct con- 
tours, an irregular shape and a homogeneous appearance (Ed 
van Beneden, 8, 699,) it soon enlarges, becomes regular, and ac- 
quires a distinct, centrally situated nucleolus, (Bischoff, 14, 50^ 
Coste 17, Lapin I'l. ii. fig. 4,) presumably by the gathering 
together of the microsomata. 
The position of the nucleus is always eccentric' so far as 
known, and approximately if not exactly the same as that of 
the egg-cell nucleus before maturation ; accordingly, the de- 
gree of eccentricity varies as the amount of yolk or deutoplasm 
being least in alecithal and greatest in telolecithal ova. In 
brief, it may be said the nucleus tends to take the most cen- 
tral position possible with regard to the protoplasm of the 
ovum. The vitelline granules are not to be regarded as pro- 
toplasm, hence their accumulation may produce a one-sided 
distension without, however, in the least disturbing the uniform 
radial distribution of the protoplasm. The nucleus is sur- 
rounded by protoplasm with few or no yolk grains ; in telo- 
lecithal ova the perinuclear accumulation is the court of pro- 
toplasm at the animal pole. 
Period of repose. After the segmentation-nucleus is formed,, 
there occurs a pause, which lasts according to observations on 
several invertebrates, from half to three quarters of an hour. 
It is probable that a similar pause ensues in the mammalian 
ovum, but there are as yet no observations to show whether it 
occurs or not. During this period the yolk expands slightly, 
unless, indeed, the expansion observed is due to the influence 
of hardening agents" and the monocentric radiation, which is 
present when the nuclei copulate, gradually fades out, and is 
> It is often stated that the nucleus lies exactly in the centre, but I have been 
» Van Beneden states that osmic acid produces an artificial expansion of the 
