764 



The American Naturalist. 



[September, 



over the pigmented hemispliere. 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. 32). 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 appekr 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 making sketches of the 

 wrinkles with a camera lucida at short intervals, as in the movements 

 of the pseudopodia of Amoeba (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 



