60 CHEMODIFFERENTIATION 



up in a highly abnormal manner (Conklin, 1931) (Fig. 23). 



From these and similar experiments it has been concluded 

 that the various protoplasmic substances may be regarded as 

 determining substances for the various organs of the embryo, 

 i.e. that they steer the development of the cells in which they 

 have become located in a certain direction. However, later 

 experiments by Dalcq (1938), Meryl Rose (1939), Reverberi 

 and Minganti (1946-49), and others, point to the fact that the 

 ascidian egg must not be regarded as merely a mosaic of 

 independently developing primordia. Complicated interactions 

 between the parts play a role in the development of these eggs 

 too. Yet, apart from these interactions, the localisation of the 

 protoplasmic substances undoubtedly is important for the devel- 

 opmental potencies of the cells. This applies in particular to 

 the mesoplasm, which does apparently determine the differ- 

 entiation of the cells in which it is present into muscle and 

 connective tissue, and is, therefore, a real determining sub- 

 stance. In this case, again, the localisation of the various cyto- 

 plasmic components after fertilisation is probably due to the 

 influence of factors in the egg cortex. 



Using histochejnical methods, Ries (1939) has proved that 

 the mesoplasm of ascidians is characterized by the presence of 

 certain enzymes (oxidases and peroxidases), as is the pole-plasm 

 of Tuhifex. This enzyme-containing protoplasm can be displaced 

 by centrifugation. Now Ries has shown that in embryos devel- 

 oping from centrifuged eggs muscle tissue originates from those 

 cells that happen to harbour the enzyme-containing plasm 

 (Fig. 24). This is a strong argument in favour of the hypothesis 

 that these enzymes themselves are responsible for the determina- 

 tion of the cells to muscular tissue. In other words, they may 

 be regarded as the determining substances in question. These 

 enzj^m.es are known to be important for cell metabolism; it is 

 probable, therefore, that they exert their influence through the 

 metabolic processes. This modification in the metabolism, in 

 its turn, may be the cause of the differentiation. 



We have seen above that the original conception of ^'organ- 

 forming substances'' supplying the building material for definite 

 organs proved to be an oversimplification. We have then re- 



