766 



The American Naturalist. 



[September,. 



The Second Cleavage. — The second furrows begin about forty or 

 forty -five minutes after the first. They start /?^om the equator and move 

 toward the dark pole, and not vice versa as in Rana, Amblystoma,^ 

 Petromyzon, and most of the amphibians. This was without exception 

 in all cases noted (some thirty-five or forty). They form a right angle 

 with the first groove, but do not always meet it at the same point 

 (Figs. 19, 20). They are attended with the same appearance, fusion,, 

 and gradual elimination of wrinkles as the first furrow, with the slight 

 difference that the wrinkles start at the equator instead of the pole, and 

 are never quite as large. This is shown in figs. 16 to 18, in which both 

 the wrinkles and the furrows can be seen approaching the pole. 



The second furrows seem to reach the first over the dark hemisphere 

 before they start toward the inferior pole. They start at about, but not 

 necessarily exactly, the same time. 



Here, too, there is the same rounding of the pigmented segments^ 

 though to a much less degree. This may be due to the fact that in this 

 instance the furrows start from the periphery, and consequently the 

 segments are more sharply defined there. 



The Third Cleavage. — Half an hour after the appearance of the 

 second set of furrows, and sometimes before they have reached the in- 

 ferior pole, there are indications of the first horizontal cleavage. As 

 seen from figs. 21-24 this occurs very much nearer the superior pole 

 than the inferior. The furrows, accompanied as before by wrinkles 

 along their edges, as seen in the figures, may start in each segment 

 either from the first or second vertical, usually, however, from the 

 second (Fig. 22). 



They move much more rapidly than the preceding cleavages, and the 

 entire process is completed in fifteen to eighteen minutes. 



In all the eggs observed the grooves started usually at the second 

 vertical in the two quadrants on the same side of the latter, and moved 

 around toward each other at the first vertical. 



They seldom met at the same point, but as a result of their forma- 

 tion the superior region was divided into four quite equal and nicely 

 rounded cells, much smaller than the four inferior ones. 



The only wrinkles formed upon the yolk area that could be detected 

 by the most careful examination are the few which occur on the yolk 

 side of this first horizontal groove. Schultze failed to detect any wrinkles 

 whatever along this horizontal furrow, although he describes the other 

 details with great exactness. This is all the more striking, because he 

 both observed and figured them upon subsequent furrows up to the 



