204 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the time of, the liberation of the medusa. During the growth period 
the chromatin of the germinative vesicle is in fine, feebly staining 
granules, and a concentration of these into larger intensely staining 
granules and beaded strands marks the prophase of the maturation 
mitosis. A decrease in size and the assumption of an ovoid shape 
by the germinative vesicle often occurs at this time. Definite asters 
with centrosomes appear, the asters increase in size, and the matura- 
tion spindle begins to form before the nuclear membrane dissolves. 
The spindle is at first parallel to a tangent at the nearest point in the 
surface of the egg, and the chromosomes, in the reduced number of 
ten or less, are arranged in a more or less complete ring at the equa- 
tor. Some of the chromosomes at least are in tetrads, which later form 
x- and y-shaped figures that suggest a longitudinal splitting. The 
spindle assumes a radial position, the asters entirely disappear, though 
the centrosomes remain, and a definite polar cell is detached from the 
egg. ‘I'he chromosomes remaining in the egg now form a more or 
less typical resting nucleus, with membrane and nuclear reticulum, 
before the second spindle appears. A second maturation spindle is 
formed and a second polar cell is detached from the egg. ‘lhe chromo- 
somes remaining in the egg form the egg nucleus, which at first is often 
composed of several distinct vesicles. ‘These may fuse, or they may 
remain distinct even till the time of conjugation of the germ nuclei. 
3. Fertilization.— At the time of entrance of the spermatozo6n 
the cytoplasm is very active, forming protuberances and papillae on 
the surface of the egg, besides definite attraction or entrance cones. 
The entrance of the spermatozo6n, which usually occurs after both 
polar cells have been formed, may be at any point in the surface of the 
egg, but more commonly near the egg nucleus. ‘The sperm head 
begins its transformation into a vesicular nucleus just within the 
surface of the egg, and sometimes several lobes or vesicles may be 
formed. ‘lhe sperm nucleus in its migration toward the egg nucleus 
often leaves a funnel-shaped “track” in the cytoplasm. The germ 
nuclei are sometimes equal, sometimes unequal in size at the time of 
conjugation, which is by apposition. Asters and centrosomes are 
usually absent at the time of apposition. Polyspermy often occurs 
and at least two spermatozoa may form vesicular sperm nuclei. 
Whether more than one sperm nucleus unites with the egg nucleus 
could not be determined. 
4. Cleavage.— The first cleavage spindle has definite polar radia- 
tions and seems to form from the cytoplasm. ‘The second cleavage 
spindle is almost completely formed before the nuclear membrane is 
