534 CELL DIVISION IN EGGS OF CREPIDULA. 



many cleavages of the macromere, it is not probable that the cleavages are 

 entirely comparable to those of normal eggs. 



If the first cleavage had already occurred when the eggs were put into diluted 

 sea water, the yolk may fail to divide in the second cleavage, so that only two 

 macromeres are present throughout the later development (figs. 137-139, 149, 

 150, 157), or one of the macromeres may divide and develop normally, while 

 the other remains undivided (fig. 140). In these cases each macromere may give 

 off micromeres in more or less normal fashion; thus one micromere of the first 

 set is formed from each of the macromeres in figs. 137-139, and these micromeres 

 are apparently normal in structure and position, though the macromeres from 

 which they came contain polyasters or multiple nuclei. However in the more 

 advanced stages of such eggs, shown in figs. 149, 150, the micromeres of the 

 different sets cannot be identified with certainty. 



In most of the eggs in which cleavage furrows have been suppressed, poly- 

 asters and multiple nuclei are present, and in many instances these are un- 

 doubtedly due to the interference of two mitotic systems, originally independent. 

 Sometimes, however, the two mitotic systems within a single cell remain inde- 

 pendent and give rise to micromeres which are in the main normal. Thus in 

 figs. 157-158 the macromeres did not divide at the second cleavage, though the 

 nuclei did. These nuclei and their mitotic figures have remained distinct and 

 have given rise to a cap of ectomeres, which are nearly normal, though frequently 

 the divisions by which they were formed have been bilateral rather than spiral; 

 thus in fig. 157 the two spindles in the one macromere, which are labeled 4C and 

 4D, are bilaterally arranged. Both figs. 157 and 158 show that the fourth quartet 

 cells 4c and 4d are formed simultaneously from the macromere CD, whereas, 

 in the normal egg, 4d is formed at the 24-cell stage and 4c at the 52-cell stage. 

 The fact that the two nuclei are in the same cell has led to the synchronizing o/ two 

 divisions, which are separated by a considerable space of time when these nuclei are 

 in separate blastomeres. Furthermore the size and structure of these cells, 4c 

 and 4d, are alike and it is probable that both are mesentoblas t s , whereas in normal 

 eggs 4d is relatively small and rich in protoplasm and is the one and only mesento- 

 blast while 4c is large and rich in yolk and is an entoblast. In the absence of a 

 division wall between C and D the typical differentiations which arise between tUse 



cells and their progeny cannot develop. . 



But while the yolk cleavage is frequently suppressed in eggs treated witn 

 diluted sea water, it is sometimes, apparently, increased, since eggs may be 

 found with more than four macromeres (figs. 143, 156). However it is probable 

 that this abnormality is due to pressure rather than to diluted sea water, 

 fig. 142 the macromeres are separated and the micromeres are crowded in be ™f\ 

 them, just as in fig. 55 of the pressure experiments. In fig. 156 it appears t 

 the egg was sub j ected to pressure in the direction of the chief axis after trie w 

 cleavage and before the fourth ; the first set of micromeres and their ae £™ ^ 

 are quite normal, the second set consists of large cells filled with yolk; tnis egg 

 more or less like figs. 56, 60, 64 of the pressure experiments. 



In 



