510 CELL DIVISION IN EGGS OF CREPIDULA. 



second set gives off a small central cell, the "tip cell," and a large peripheral one; 

 while the left half of each of these micromeres reverses this, giving off a small 

 peripheral cell from a large central one; there is thus formed an "ectodermal 

 cross" with its center at the animal pole and with an arm running out over 

 each macromere and ending in the "tip cell" which is larger in quadrant D than 

 in any of the others (see stippled cells, fig. 43). In all of these respects the % 

 eggs shown in figs. 43 and 44 are entirely typical though they lack all the cells 

 of one quadrant. Finally in the partial egg shown in fig. 43 the macromere 

 4D has given rise to the mesentoblast, 4d, which has divided into right and left 

 halves, M 1 and M 2 , in a manner entirely typical; on the other hand when the 

 macromere D is lacking no mesentoblast cell is formed. Other partial eggs 

 caused by diluted sea water are shown in figs. 135, 144, 151, 155. These also 

 illustrate the principle that the development of isolated blastomeres is entirely 

 typical so far as the cleavage of each quadrant is concerned. The cleavage from 

 the 4-cell stage on is, in the language of Roux, "a mosaic work consisting of four 

 independently developing vertical pieces." These results are in essential agree- 

 ment with those reached by Roux (1892), Chabry (1887), Crampton (1896), 

 Wilson (1904), Conklin (1905) on the cleavage of isolated blastomeres in eggs 

 of animals having "determinate" types of cleavage. Of course where individual 

 blastomeres are not peculiar in size and quality it is not possible to determine 

 to what extent the cleavage of isolated blastomeres is atypical. In Crepidula 

 the individual peculiarities of the blastomeres are so marked that we have here 

 one of the best possible opportunities for the study of this problem. 



III. Effects of Pressure on Cleavage. 

 (Plates XLVI, XLVII, XLVIII. Exps. 849-854, 884-886, 893-920, 1000-1004.) 



Another method of studying the determinism of cytoplasm and nuclei in the 

 cleavage stages is to subject the eggs to pressure so that the spindle axes and 

 cleavage planes are turned out of their normal positions. Such experiments 

 were performed by Pfltiger, Roux, Born, and O. Hertwig, on amphibian eggs; 

 by Driesch, Morgan, and Ziegler, on echinoderm eggs; by Ziegler, on ctenophore 

 eggs; and by Wilson, on the eggs of Nereis, etc. In this way the early cleavage 

 furrows may be displaced from their normal positions so as to form linear series 

 or flat plates of cells, the distribution of ooplasmic substances (yolk, cytoplasm, 

 etc.) to the blastomeres may be greatly changed and the nuclei which in normal 

 eggs would have gone into micromeres may be caused to go into macromeres. 

 Driesch and Hertwig in particular have maintained that these pressure experi- 

 ments demonstrate the totipotence of the cytoplasm and karyoplasm, since 

 normal development may result from eggs in which both of these plasms are 

 abnormally distributed. Nevertheless Driesch has shown, and most of the other 

 investigators named above confirm this conclusion, that the structure of the 

 cell protoplasm differs in different axes, and that the cleavage of isolated blasto- 



