506 CELL DIVISION IN EGGS OF CREPIDULA. 



plane while the other is far away from that plane. In fig. 9 the entire spindle 

 has been moved into one of the daughter cells, and the division plane between 

 the cells has cut off a cell containing two daughter nuclei from one containing 

 none. In this case a Zwischenkorper or mid-body is present in the plane of 

 division, although the entire mitotic figure lies to one side of that plane. This 

 seems to indicate that the mid-body may be formed, or at least may persist, 

 independently of the mitotic figure. 



Other abnormalities of division which are probably due to pressure, are found 

 in figs. 22-25; here the number of macromeres is increased from four to six, seven 

 or eight, and this is probably due, as experiments show, to pressure in the vertical 

 axis during the third cleavage. Such increase in the number of macromeres is 

 not to be interpreted as the result of an "anachronism of cleavage" (Roux), i. e., 

 to the substitution of a later cleavage for an earlier one, nor are the eight blasto- 

 meres so formed comparable to those of the 8-cell stage of the normal egg. 

 On the contrary this cleavage is one not represented in normal eggs but is a new one 

 which has been intercalated, and though in point of time it is the third cleavage, it is 

 morphologically like the first and second cleavages and not like the third cleavage of 

 the normal egg; it gives rise to no micromeres and serves merely to increase the number 

 of macromeres to a number larger than four. In subsequent cleavages each one 

 of these macromeres gives rise to three micromeres (ectomeres) exactly as if it 

 were one of the four normal macromeres. From the directions of these divisions 

 and from the size and quality of the resulting cells it is perfectly evident that the 

 micromere formation from each of these macromeres takes place exactly as in a 

 normal egg. Each macromere which reaches the animal pole gives off three 

 micromeres (ectomeres). In figs. 23 and 24 where six macromeres reach the 

 animal pole, and one lies far below, near the vegetal pole, there are six micro- 

 meres of the first set ("quartet"); in fig. 24 five of the macromeres are dividing 

 in a Isotropic direction to form the micromeres of the second set, and it is evident 

 from the position of the nucleus and sphere that the sixth and smallest macro- 

 mere will also give rise to a micromere of the second set when it divides. In fig. 

 25 it is evident from the positions of the spindles in the five macromeres that the 

 third set of micromeres are being formed, and although it is not possible in this 

 figure to identify certainly all the micromeres of the first and second sets, it is 

 probable that there are six micromeres of the second set, and six of the first, 

 each of the latter having budded off in Isotropic direction a small "turret cell." 

 In fig. 26 there are but three macromeres, two of which have given rise to three 

 sets of micromeres, whereas the third macromere (D) is dividing for the first time. 



In several cases in which more than four macromeres are present the nuclei 

 of some of the cells are either irregular in form or multiple in number. Thus 

 in fig. 22 there are several scattered nuclei in four of the cells (three at the animal, 

 and one at the vegetal pole), and in fig. 24 there is a triaster in one cell and 

 double nuclei occur in two micromeres and in one macromere. These are prob- 

 ably karyomeres or partial nuclei, due to polyasters, such as are seen in fig. 16, and 



