1904.] NATURAL SCIEXCES OF PHILADELPHIA. 693 



second. Neither of them have been seen to divide. The polar bodies 

 in Dinophilus, as in all animals, mark the animal pole, but are, however, 

 not a reliable means of orienting the later stages, since through some 

 cause they tend to become displaced and are ultimately taken into the 

 cells over which they happen to lie. In fig. 8, for example, the first 

 polar body is already sinking into the cell Ic, while the second is still 

 free. 



In fig. 1, beneath the polar bodies lies the female pronucleus, formed 

 of four nuclear vesicles, each vesicle resembling a small nucleus in 

 appearance, having a distinct bounding membrane and containing 

 small granules of chromatin of varying sizes. Below the vesicles, and 

 contaimng them as in a cup, is a large hemispherical aster. Somewhat 

 below and to the right of the center of the ovum lies the male pro- 

 nucleus with its accompanying aster, which lies on the vegetal pole 

 side of the nucleus. 



The stage next studied is represented in fig. 2, where the two pro- 

 nuclei are seen to have come together. The male pronucleus is prob- 

 ably the large bilobed vesicle which lies on the side of the nucleus 

 toward the vegetal pole; the female pronucleus is probably represented 

 by the eight smaller vesicles on the other side. The ovum is in the 

 early prophase of division, a large and deeply staining aster being on 

 each side of the nucleus, though no spindle fibres can yet be distin- 

 guished. 



(2) Primary Cleavages. 1-4 Cells. 



The spindle for the first cleavage is shown in fig. 3. The cell body 

 has elongated and the spindle is in the anaphase, while the unequal 

 character of this cleavage is clearly indicated by the inequality in the 

 diameter of the two asters as well as by the eccentric position of the 

 spindle. The spindle is inclined at a slight angle to the horizontal 

 plane, the end which is to form the smaller cell being the lower. The 

 explanation of this is not clear; possibly the spindle was oscillating, 

 or perhaps the position of the spindle may have something to do with 

 the beginning of spiral cleavage. Unfortunately this is the only ovum 

 which I have seen in which the spindle is in either metaphase or ana- 

 phase, so that it is not absolutely certain that this oblique position is 

 normal. The plane of this division passes through the animal pole, 

 but lies to one side of the pole opposite. The products of this division 

 (fig. 4) are very unequal, the cell C-D greatly exceeding A-B in size. 



Immediately after division both cells prepare to divide again, but 

 the two halves of the second cleavage are not simultaneous, C-D divid- 



