538 William A. Hilton. 



segmentation was recognized — a very slight depression near the 

 center of the animal pole, followed by a very narrow groove which 

 was deepest in the center and became less deep as the edge of the 

 light part was approached. For quite a time this very minute crease 

 in the egg seemed to remain shallow and not more extensive than the 

 edge of the light polar disc. Figs. 1 and 2 show views of an unseg- 

 mented egg from above and from the side. Fig. 3 an early two-cell 

 stage from above. This first line of cleavage may remain straight 

 for some time or become slightly curved in its middle portion. It 

 may become broader in its central portion in a short time and quite 

 decided bead-like irregular cavities may develop along the line of 

 cleavage, or very often one or both ends of this line near the edge 

 of the light polar cap may develop club-like outlines due to the 

 separation of the yolk to form deep clefts. These enlarged ends of 

 the first segmentation plane seem to limit its extension about the 

 egg for some time. 



After about four hours, at the light pole of the egg and at right 

 angles to the first division, the second segmentation plane makes its 

 appearance as a very slight furrow. It is much as the first was 

 in the beginning, but now the first cleavage line is much more promi- 

 nent than the second, although perhaps the first cleavage lines do not 

 even yet extend beyond the light pole. At first the two cleavage 

 planes cross each other at right angles, but later there is a shifting 

 of the partial blastomeres or of some of the granules composing them 

 and the two first planes of cleavage have a small portion of their 

 extent in common. Fig. 4 shows a four-cell stage in which the 

 second division plane has just appeared and Fig. 5 shows the same 

 egg twenty minutes later. The lighter lines in each case indicate 

 second division planes. The first division plane in four-cell stages 

 as a rule is much deeper than the second, but Fig. 7 is an exception. 

 The division planes may at quite an early time present a beaded 

 appearance due to small irregularities in the yolk along the line of 

 their course. From the time the first cleavage plane a])pears, it be- 

 gins to gradually but slowly extend itself about the egg, either mostly 

 from one side or from both sides, but this extension is so slow and 

 other divisions are so well along before any marked appearance of 



