CELL DIVISION IN EGGS OF CREPIDULA. 507 



such an egg would probably divide at once into seven or eight cells, the number 

 of macromeres in this case being due immediately to abnormalities in the mitotic 

 figure rather than to pressure. In this case the initial causes of these triasters 

 and polyasters can only be conjectured; probably cell division was suppressed 

 by pressure, while centrosomal and nuclear division went on, as will be explained 

 later. 



A few other types of abnormal cleavage occurring in nature have been ob- 

 served. The eggs shown in figs. 6, 7, 8, have not divided into several macro- 

 meres, but the yolk has remained undivided while two separate micromeres have 

 been cut off at the animal pole of the single macromere. The mitoses by which 

 these cells were formed were abnormal, as is shown by the character of their 

 nuclei, but the positions of the centrospheres, and the relation of the cells them- 

 selves to the macromere, as well as the position of the spindle in fig. 6, indicate 

 that these small cells may correspond to the first and second micromeres of the 

 normal egg. If this be true we have here another evidence of the fact that 

 the number of micromeres is proportional to the number of macromeres, in every 

 case three and only three micromeres being cut off from each macromere, i. e., with 

 one macromere the micromeres are formed in sets of one, with two in twos, with four 

 in fours, with six or eight in sixes or eights, etc. In this connection it is 

 interesting to note that Bigelow (1902) found in the egg of Lepas that the single 

 macromere of that egg gave rise successively to one first, one second and one 

 third ectomere and then to one mesomere, and he concluded that such a type of 

 cleavage might be derived from the "quartet" type by the suppression of the 

 first and second cleavages of that type. 



Fig. 18 shows an egg in the 4-cell stage in which the nuclei are entirely lacking 

 in two of the cells, although other parts, such as sphere and mid-body, are present. 

 I do not know how such an abnormality has arisen but it may be that the nuclei 

 were extruded after the division of the cells. On the other hand fig. 21 shows an 

 egg in which the nuclei continued to divide in normal rhythm although the 

 cell body did not, three of the spindles of the third cleavage being found in one 

 cell, without proper orientation, while the spindle in the other cell is normal in 

 position. 



Finally figs. 27, 28, 29 represent a few eggs and embryos from a great number 

 in which the yolk mass or endoderm was not overgrown by the micromeres, thus 

 giving rise to exogastrulse. The lot of eggs showing these abnormalities was 

 found in a place where the sea water was diluted with fresh water, and the 

 experimental study of the effects of diluted sea water on the eggs shows that this 

 is the probable cause of these abnormalities. 



These are the only types of abnormalities which I have found among the 

 many thousands of normal eggs which I have studied; among these, however, 

 are representatives of several forms of abnormalities of cell division which have 

 occurred in my experiments. 



