84 



CLEAVAGE AND DIFFERENTIATION 



direction of the longest axis of the cytoplasm of the cell.^ The 

 distortion occasioned by the glass plates causes the third cleavage 

 plane to form meridionally instead of latitudinally^ (figs. 35, 36). 

 One of the consequences of the experiments of forcing eggs to 

 cleave under compression is that the normal distribution of the 



Fig. 35 

 Disarrangement of cleavage by pressure in sea-urchin eggs, a-d, Normal 

 cleavage. e,f and i-k. Flat plates arising from cleavage under pressure, g, h, Sub- 

 sequent cleavage of/ when released from pressure. /, The same for k. The late 

 cleavage stages are drawn with the vegetative pole uppermost. The disarrangement 

 of the nuclei does not prevent the development of normal plutei. (After Driesch, 

 from Morgan, Experimental Embryology , Columbia University Press, 1927.) 



cleavage nuclei is altered, but subsequent development is normal 

 in spite of the fact that a number of nuclei find themselves in 

 blastomeres other than those in which they would be situated in 



^ Although this rule is of very general application, there are some notable ex- 

 ceptions to it. For instance, cleavage in the star-fish Patiria occurs in relation to 

 the polarity of the egg, whether original or induced by operation (see p. 313), 

 even when the egg is deformed by pressure (Taylor and Whitaker, 1926). Other 

 exceptions are found in the first cleavage of Ascaris eggs, and in the divisions of 

 the cells forming the germ-bands of Crustacea (see Jenkinson, 1909B, p. 34). 



- Frog, Hertwig, 1893; sea-urchin, Driesch, 1893. 



