186 bulletin: museum of comparative zoology. 



germinative vesicle at the end of the growth period before the dissolu- 

 tion of the nuclear membrane takes place, part of it entering the retic- 

 ulum and part, perhaps, the nuclear sap. While not chromatin, it 

 appears to be capable of transformation into chromatin, and so may 

 perhaps serve as a storehouse for material needed in the formation of 

 the chromosomes. Its disappearance is by a loss of liquid substance 

 or by fragmentation, or by both. 



3. Polar-Cell Formation and Fertilization. — Just before the time 

 of polar-cell formation the egg is rounded and without pseudopodia; 

 the germinative vesicle lies close to the periphery of the egg, its nucleo- 

 lus has disappeared and its chromatin is finely divided and distributed 

 along the nuclear reticulum. A decrease in the size of the germi- 

 native vesicle takes place, as a comparison of Figures 45 and 46 with 

 Figure 48 shows. 



Beginning at this time, or somewhat earlier, the reticulum of the 

 germinative vesicle again stains intensely with nuclear dyes. The 

 chromatin is assembled at the nodes of the network in masses, usually 

 irregular in shape and granular in appearance, which suggests the 

 grouping of smaller granules into the larger mass. The appearance 

 in the fixed egg is well shown in Figure 48, though the germinative 

 vesicle at this stage is usually not so far from the surface of the egg. 

 Often the cytoplasm around the nucleus is finely granular and desti- 

 tute of the vacuoles which are present in the rest of the cytoplasm. 



The history of the subsequent changes has been made out in part 

 only, since certain stages could not be found. The most characteristic 

 condition found when the time of polar-cell formation approaches is 

 that shown in Figures 49 and 50. The nucleus has become ovoidal, 

 the long axis having a radial position. The outer end of the nucleus 

 is often sharply pointed, and this seems to represent a later stage than 

 the more rounded condition shown in Figure 49. Almost always at 

 this stage radiations are present at the outer end of the germinative 

 vesicle, and the surface of the egg itself is often raised into a more or 

 less conical elevation at this point. The centre of the radiations is 

 always between the germinative vesicle and the surface of the egg; 

 in no case were centrosomes found, nor were radiations seen at any 

 other part of the nucleus. Since polar cells were apparently not 

 present, it seems probable that this is a prophase of the first matura- 

 tion figure, though a somewhat similar condition in Pennaria (Figs. 14, 

 15) represents a stage after the first polar cell has been formed. There 

 is no evidence upon which to determine whether there is a division 

 of this aster to form two centres, or whether the spindle centres arise 



