764 The American Naturalist. [September, 
over the pigmented hemisphere. The ends may progress at a uniform 
rate, or one may exceed the other in rapidity as in Petromyzon (Eycles- 
hymer, loc. cit.). 
At the first appearance of the furrow it is very shallow and perfectly 
smooth, and extends about 0.2 mm. on either side of the pole. It be- 
gins to deepen in two or three minutes, and at the same time minute 
wrinkles appear on either side (Plate I, figs. 1,5, 7). If the eggs be 
placed upon a black surface in the sunlight these wrinkles are seen very 
distinctly as fine lines radiating from the pole (Plate II, fig. 31). The 
number and arrangement of these lines is not constant or definite either 
at this, or any subsequent period. The suggestion at once presents 
itself, that these wrinkles may be the foreshadowing of subsequent seg- 
mentations, and one egg was obtained in which the wrinkles seemed 
especially significant in their arrangement (Plate II, fig. 82). The 
entire lack of regularity in the size and arrangement of the wrinkles 
would, however, preclude any such idea, since the subsequent segmenta- 
tions are very regular. 
As the furrow gradually deepens and extends toward the yolk two 
changes are noticed 
1. New wrinkles appear along its sides ; these do not all radiate from 
the pole, but are inclined toward it at greater or smaller angles (Figs. 
6, 8, 11). 
2. The radial wrinkles first formed at the pole change considerably. 
Very fine and delicate at first they coalesce gradually into a few larger 
and deeper ones, which are sharply defined. 
These fused ones may or may not occupy the position of one of the 
antecedent ones (Figs. 2, 3, 33, 34,35). As the furrow progresses the 
number and position of the wrinkles changes constantly, new ones being 
formed and old ones disappearing. This is especially true of the finer 
ones ; some of the larger fused ones near the pole remain quite constant 
(Figs. 10-15). 
This continual changing is best seen by pea ie sketches of the 
wrinkles with a camera lucida at short intervals, as in the movements 
of the pseudopodia of Amæba (Figs. 1-4 and 10-15). 
The whole appearance thus far is exactly as if the egg were covered by 
a very thin, but firm membrane, which was gradually pulled in toward 
the center at the groove. The remainder of the sphere being perfectly 
even, and with no chance for “ give” at any point, in consequence of 
the uniform tension, the edges of the groove would necessarily be 
-wrinkled, the wrinkles becoming more and more prominent as the 
groove deepened. The whole process takes place so slowly that the 
