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 V-shaped figures that suggest a longitudinal splitting. The 

 spindle assumes a radial position, the asters entii'ely disappear, though 

 the centrosomes remain, and a definite polar cell is detached from the 

 egg. The 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. The 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 spermatozoon 

 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 spermatozoon, 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. The 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 



