THEORIES OF DEVELOPMENT 2$$ 



which Driesch has made of the phenomena of development. His ana- 

 lytical theory rests on the dictum that the prospective value of each 

 blastomere is a function of its position in the whole. By " function " is 

 meant " a relation of dependence of a general unknown kind." This 

 idea is connected with the following one, viz. that any blastomere 

 could be interchanged with any other one without altering the end- 

 result. A few elementary processes are supposed to be "given" in 

 the structure, or in the composition of the egg. Each elementary 

 process is the outcome of a cause, and each elementary process must 

 release the succeeding causes, i.e. if the organization of the phase 

 A is present, one of the causes of the next phase B is also then present. 

 The first elementary process is the cleavage, that is initiated ("aus- 

 gelost ") by the fertilization. After a fixed number of divisions has 

 taken place, the cleavage process comes to an end. It has led to the 

 production of a number of cells having similar nuclei but having a 

 different plasma structure, and the result is the blastula stage. Or- 

 gans whose formation starts from the blastula stage arc called primary 

 organs ; the archenteron, the mesenchyme, the ciliated band, and the 

 mouth of the sea-urchin embryo belong to this class. Secondary 

 organs are those that arise from the primary ones, as the ccelom sacs, 

 for instance, in the sea-urchin embryo. The primary organs are 

 started by the setting free("Auslosung") of a new elementary process 

 in the blastula, and later the secondary organs are started by new ele- 

 mentary processes that arise in the gastrula, which cannot appear until 

 the gastrula stage itself is present as a starting-point. In other 

 words, the elementary processes that are " given " in the egg can 

 only come into action, or be set free after a certain stage has come 

 into existence. This means that we must think of each organ that 

 responds to a stimulus as having the possibility of receiving that stimu- 

 lus, and also of answering to it. Even in inorganic nature every reac- 

 tion must depend on a specific receptiveness and a specific answer. 

 Driesch supposes that the receptivity is in the protoplasm, and the 

 power to respond is in the nucleus of each cell. In this way he 

 attempts to meet the difficulty that the nucleus is, in every cell, the 

 bearer of the totality of all the " Anlagen" ; but inasmuch as it is sur- 

 rounded by a specific plasma, it is in a position to receive only cer- 

 tain stimuli, and can therefore only respond to certain causes. 



In the specific nature of the cytoplasm of the cell lies the pro- 

 spective potence of every organ, and the possibilities of each cell are 

 limited by its plasma ; the cell becomes more and more limited as 

 development proceeds. It may be said, therefore, that in the course 

 of development the cells become actually limited in their possibilities, 

 although they may still retain within themselves, in the nucleus, the 

 potentialities of the entire organism. 



