?4 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



gure 166C will be displaced horizontally by the applied pressure, as shown 

 1 figure 165F. If pressure is released at this stage, the mitotic spindle again 

 beys Hertwig's rule and forms in the long axis of the cytoplasm which is 

 ow vertical in position. As a result, upper and lower cells are formed, as 

 1 figure 166G. The original destiny of the nuclei in the cells producing ecto- 

 erm is shown in white circles; that for the cells destined to produce mesen- 

 lyme, entoderm, and ectoderm is shown in black (figs. 163, mesomeres; 

 66C, D). As shown in figure 166G, there is a mixture of these nuclei after 

 le pressure is released. Regardless of this redistribution of nuclei, develop- 

 lent proceeds almost normally. Development thus appears to be governed 

 y the presence of special ooplasmic substances contained within the respective 

 lastomeres (figs. 163; 166A-D). 

 The evidence from the foregoing experiments suggests the conclusion that 

 le nuclei in the early blastomeres are qualitatively equal. Consequently, this 

 ody of experimental evidence is antagonistic to the older view of Weismann, 

 ho held that differences in the various parts of the developing organism 

 ■e to be attributed to "differential nuclear divisions" whereby different he- 

 ;ditary qualities (i.e., biophors ) are dispersed to different cells. To quote 

 om Weismann (1893, p. 76): 



Ontogeny depends on a gradual process of disintegration of the id of germ- 

 lasm, which splits into smaller and smaller groups of determinants in the devel- 

 pment of each individual, so that in place of a million ditlerent determinants, of 

 hich we may suppose the id of the germ-plasm to be composed, each daughter-cell 

 I the next ontogenetic stage would only possess half a million, and each cell of 

 le following stage only a quarter of a million and so on. Finally, if we neglect 

 assible complications, only one kind of determinant remains in each cell, viz., 

 lat which has to control that particular cell or group of cells. 



E. Quantitative and Qualitative Cleavages and Their Influence upon 



Later Development 



One of the earliest students of the problem of the developmental possi- 

 ilities of isolated blastomeres was Hans Driesch (1891 and 1892). In these 

 jblications, Driesch offered the results of experiments in which he shook 

 Dart the early blastomeres of the sea urchin and studied their development, 

 riesch found that the two blastomeres resulting from the first division con- 

 nued to divide, and as though the other blastomeres were present. The 

 rst division of the isolated blastomere was meridional, as if it had retained 

 Dntact with its mate of the two-cell stage. The next division was latitudinal, 

 so, as if it had retained contact with its original mate. Ultimately each iso- 

 ted blastomere developed into swimming blastulae of half the normal size, 

 he four blastomeres of the four-cell stage were similarly isolated. Here, 

 Iso, each divides as if it were part of the whole, and free-swimming blastulae 

 2velop. However, later development is imperfect or definitely abnormal. 



