284 BULLETIN OF THE 



portion of protoplasm has the form of a dumb-bell, the rodlike middle 

 portion being the part which contains the " fissure " before its disappear- 

 ance. From each of the spheres of the dumb-bell stretch out on all 

 sides rays of clear protoplasm, between which rows or wedge-shaped 

 masses of granular yolk are embraced. The rays are usually straight, 

 sometimes slightly bent with the concavity directed toward the centre 

 of the egg, and give the figure the appearance of two pale suns united 

 by a long middle piece. The lengthening of the figure continues till the 

 middle piece is more than half the length of the egg. 



The formation of the dumb-bell figure begins with the radial arrange- 

 ment of the yolk granules about the tips of the broadly spindle-shaped 

 nucleus, i. e. only when the latter begins to lengthen. The pale rays 

 intervening between the rows of granules become more conspicuous, and 

 their bases unite to form the head of the dumb-bell, while the " middle 

 piece " is forming. 



These phenomena Auerbach explains as follows. The nucleus per- 

 ishes, and during the lengthening of the nuclear cavity the sap which 

 fills it penetrates between the molecules of the neighboring protoplasm, 

 forcing the yolk granules out of it. The rays about the tips of the nu- 

 cleus are the (physical) expression of the courses along which the fine 

 streams of nuclear sap penetrate the protoplasm. The distribution takes 

 place from these tips for two reasons : because the tip of the nuclear 

 cavity presents a greater amount of surface, as compared with the con- 

 tents, than any other portion, — hence the point of least resistance ; and 

 because the sap, on account of the lengthening of the nucleus, is in 

 motion toward these two points. Subsequently, the resistance of the 

 lateral walls of the nuclear cavity is so far overcome as to cause the re- 

 cession of the granules from a thin layer of the adjacent yolk substance. 

 The nucleus, however, is not the active element. The protoplasm acts 

 on the passive nuclear sap by changing the form of the nuclear cavity, 

 and by imbibing the sap. In place of a nucleus there is now in the 

 yolk a peculiarly shaped territory free from granules, in which all the 

 substance of the nucleus is dissolved, — the Icaryolytic figure. 



The formation of this figure is followed by the segmentation of the 

 yolk. The latter is accomplished by a furrow, which, advancing from 

 one side only, passes through the yolk, or possibly sometimes by an 

 annular constriction. Soon after the beginning of the segmentation 

 there appears a vacuole at each of two corresponding points in the stem 

 of the karyolytic figure near the cleavage plane. These are at first 

 small, being irregularly and indistinctly outlined. They increase and 



