INDIVIDUALITY OF THE GERM-NUCLEI. 25! 



however, that the original germ-nuclei do not always lie in the 

 same horizontal plane (Fig. i). Moreover, the stage with 

 newly-formed daughter-nuclei was not represented in horizontal 

 sections, which are naturally most favorable for the discovery 

 of nuclei consisting of vesicles lying in the same horizontal 

 plane. There is no external landmark by means of which this 

 stage may be recognized, and it is passed through very rapidly; 

 in the making of horizontal sections the available material was 

 exhausted without rinding the precise stage desired. 



In the study of the subsequent behavior of the nuclei the early 

 cleavage furrows serve as convenient landmarks for orientation, 

 and it is necessary to keep in mind the elementary fact that the 

 second cleavage furrow forms at right angles to the first, and 

 that a similar alternation in the direction of successive cleavage 

 furrows is characteristic of the entire process of cleavage. Of 

 more immediate importance to us is the correlated fact that the 

 second nuclear division occurs in a direction at right angles to 

 the first nuclear division, and that this alternation in the direc- 

 tion of successive nuclear divisions is repeated throughout cleav- 

 age. To be sure, this is strictly true only in the early cleavage 

 stages, for there is increasing irregularity in the direction of both 

 cleavage furrows and nuclear divisions throughout the later stages 

 of cleavage. Incidentally, it should be noted that nuclear di- 

 vision always precedes the formation of the corresponding 

 cleavage furrow. 



In preparation for the second nuclear division, the two asters 

 of a single daughter-nucleus take up positions on opposite sides 

 of the nucleus, in the same horizontal plane and with a line 

 connecting them parallel to the first cleavage furrow which is 

 now usually in process of formation (Figs. 12-20; 23-26). In 

 case the two nuclear vesicles of a single daughter-nucleus are 

 not already ranged midway between the two asters, they rotate 

 to this position (compare Figs. 15-16 with Figs. 17-18). Typi- 

 cally, the two nuclear vesicles come to lie in the same horizontal 

 plane (Figs. 17-20, 23-26) ; but this is not the invariable position, 

 for in three different eggs they were found to lie one above the 

 other, as exemplified in Figs. 21 and 22. In any case the final 



