220 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



siderably enlarged. The male pronucleus has also increased in size and 

 followed the lead of its attraction sphere toward the centre of the egg. 

 In the dorsal half of the section is seen the female pronucleus, already 

 grown to considerable size. In it can be discerned small chromatic 

 granules, and behind it and deeper in the section the female archoplasm. 

 This archoplasm seems to be much less energetic than that of the male 

 element, for its influence is scarcely perceptible, even on the portion of 

 the egg in which it lies, and it does not appear to modify either the shape 

 or course of the female pronucleus, which, as we shall see, moves toward 

 the male archoplasm leaving its own behind. The polar globules repre- 

 sented at the margin of this section do not as a matter of fact occur in 

 that position, but at the margin of the preceding section. If that sec- 

 tion were properly supei'posed on this, the polar globules would lie over, 

 but a little to the left of the female pronucleus. 



A stage semewhat later than the one just described, though found in 

 the same lot of eggs, is shown in Plate II. Figures 7-10, which represent 

 the fourth, seventh, tenth, and twelfth sections respectively of a series 

 of sixteen. In Figure 7 is seen the male pronucleus with its archoplasm 

 now divided ; in Figure 8, the female pronucleus ; in Figure 9, the female 

 archoplasm; and in Figure 10, the polar globules marking both the 

 centre of the future dorsal surface of the embryo, and the point from 

 which the female pi*onucleus starts in its journey through the egg toward 

 the male pronucleus. The position of these various bodies with relation 

 to one another can be most clearly illustrated by two reconstructions 

 (Figs. 11 and 12) upon planes perpendicular to the plane of sectioning 

 and at right angles to each other. Suppose the sections piled one above 

 another in their original order and position, the first section of the series 

 being uppermost and the egg thus reconstructed to be viewed as a trans- 

 parent object in the direction of the arrow at the left of Figure 7. One 

 would then see the appearance shown in Figure 11, which is a projection 

 of the egg and the most important bodies in it upon a plane parallel to 

 the line a b (Fig. 7), and perpendicular to the plane of Figure 7. 



If the egg be viewed in the direction of the arrow at the top of 

 Figure 7, one gets the appearance shown in Figure 12, which is a pro- 

 jection upon a plane parallel with the line a' b' in Figure 7, and perpen- 

 dicular to the plane of that figure. 



A comparison of the stage under discussion with that represented in 

 Figure 5 shows that considerable changes have occurred in the interval 

 between them. The male pronucleus (Fig. 7) has grown to much 

 greater size and contains several conspicuous chromatic granules. In- 





