xv] DIFFERENTIATION 251 



angles to both. The fourth cleavage is a differential cleavage. 

 Four micromeres appear at one pole ; the cells of the opposite 

 hemisphere divide equally. This cleavage is a critical one, 

 for the micromeres show in the normal egg where the diges- 

 tive tract will develop. The micromeres become the mesen- 

 chyme 1 ." 



In the centrifuged eggs it was determined that the arch- 

 enteron develops at the micromere pole, and that the micro- 

 meres lie opposite to the funnel in the envelope, independ- 

 ently of the secondarily induced axis. " In other words, 

 while the first three cleavages come in with respect to the 

 stratification, the fourth or differential cleavage comes in 

 with respect to the original egg-axis." Normal plutei were 

 obtained from such eggs, in some of which the red pigment 

 was contained in one region, in others, in other regions. 



MORGAN'S experiments on Cumingia and other forms, 

 and BOVERI'S on Ascaris also lead to the same conclusion, 

 that in general the differentiation of parts in the embryo 

 is not dependent on the visible substances of the egg, but 

 on an invisible polarity that is independent of the substances 

 which can be moved or transposed by the centrifuge. 



CON KLIN (1917) explains this polarity as being due to the 

 highly viscid or elastic " spongioplasm " of the egg, "which 

 always tends to come back to its normal form if it is stretched 

 or distorted, and which probably differs in structure in 

 different parts of the egg and in different stages of develop- 

 ment." And that the visible substances which distinguish 

 the regions of the egg, especially in Tunicates (e.g. in Styela 

 mentioned above), are not in themselves "organ-forming" is 

 further indicated by the work of DUESBERG and of HIRSCH- 

 LER, quoted by GATENBY (1919^) and of GATENBY himself, 

 for these investigators find that the various regions of the 

 1 MORGAN, 1909, p. 157. 



