468 The American Naturalist. [June, 
opaque than the surrounding portions of the vitellus. After a 
few moments, this area grows less distinct. It finally appears 
elongated. Very soon this lengthening results in two light 
spots which are inconspicuous at first, but which increase in 
size and distinctness, and presently become oval. If the out- 
line of the &g^ be carefully watched, it is now seen to lengthen 
gradually in a direction corresponding to the line which joins 
the spots. As the latter enlarge, the lengthening of the ovum 
increases, though not very conspicuously. Soon a slight flat- 
tening of the surface appears just under the polar globules, 
the flattening changes to a depression. (Fig. i) which grows 
deeper and becomes angular. A little later the furrow is seen 
to have extended around on the sides of the yolk as a shallow 
depression, reaching something more than 
• half way toward the vegetable or inferior 
pole, and in four or five minutes after its 
appearance the depression extends com- 
pletely around the yolk. This annular 
constriction now deepens on all sides, but 
most rapidly at the animal pole ; as it 
deepens it becomes narrower, almost a 
Fig. I. Ovum of Umax Assure. By the further deepening of the 
campestris, during the constriction on all sides there are formed 
first cleavage. Magni- 
fied 200 diameters. The two equal masses, connected by only a 
'n.^AfterrrMark." blender thread of protoplasm, situated 
nearer the vegetative than the animal pole, 
and which soon becomes more attenuated and finally parts. 
The first cleavage is now accomplished. Both segments un- 
dergo changes of form, they approach and flatten out against 
each other, and after a certain time themselves divide. 
PrimltiTe type of segmentation. In the lower animals there is 
not found that excessive amount of deutoplasm in the ovum 
which is so characteristic of the vertebrates, and in their ova 
we have what is undoubtedly the earlier and more primitive 
type of segmentation. In these cases the cleavage extends as 
in the ^^^ of Limax, (see above) through the whole of the 
dividing cell. The two cells first produced are almost if not 
