168 bulletin: museum of comparative zoology. 



is independent of the concentration of the chromatin. No centrosomes 

 or radiations have yet appeared in connection with these germinative 

 vesicles. Before the chromosomes begin to form, the germinative 

 vesicle becomes smaller, and often ellipsoidal, and the cytoplasm forms 

 a special zone about it (Figs. 5, 6). The conditions shown here, 

 especially the presence of the cytoplasmic envelope, are those which 

 immediately precede the formation of the maturation spindle, for they 

 occur during the following stages (Fig. 7) when the spindle is forming, 

 but are not present in the earlier germinative vesicle (Figs. 3, 4). It 

 should be said that the ellipsoidal shape of the germinative vesicle 

 may be found some time before the spindle appears, eggs killed ten 

 hours before the time of discharge sometimes having this form (Fig. 2). 

 Figures 5 and 6 show only a slight concentration of the chromatin, 

 and neither centrosomes nor radiations. The next older stage found 

 (Fig. 7) already possessed centrosomes with well marked astral rays 

 in the cytoplasm, and the maturation spindle was beginning to be 

 formed. The cytoplasmic envelope surrounding the germinative 

 vesicle was less evident than in the preceding stage. The membrane 

 of the vesicle was deeply indented opposite one aster, and slightly so 

 opposite the other. This may be taken as evidence that the matura- 

 tion spindle is formed, in part at least, from the cytoplasm, and that it 

 may aid in the dissolution of the nuclear membrane. 



The next stage which I have found shows the first maturation spindle 

 with the chromosomes at its equator. In this stage the spindle is 

 approximately tangential to the nearest point of the surface of the 

 egg, though one pole is sometimes a little closer to the surface of the 

 egg than the other. The polar radiations are less distinct than form- 

 erly, though the centrosomes are still present. Both centrosomes and 

 radiations were at first overlooked, and in a previously published note 

 (G. T. Hargitt, :09) they were stated to be absent. Figures 8a, 8b, 

 9a, 9b, 10a and 10b are nearly polar views of the equatorial-plate region 

 of three eggs, the chromosomes in each case extending over two sections. 

 Figure 11 (Plate 2) is a similar view of a fourth egg, though here the 

 chromosomes occupied three sections. In Figures 9 and 10 several of 

 the chromosomes are seen to be in the tetrad condition, and a splitting 

 of some of them seems to have begun. Others appear as short rods, 

 but are so small that the direction in which the division occurs cannot 

 be accurately determined. Figure 11, however, makes this somewhat 

 clearer. Here are found bodies in rod-, x- and v-shaped conditions, 

 as well as some which have a roundish or irregular form. The im- 

 pression made by the appearance of the chromosomes is that the rods 



