OUTLOOK. 673 



for the regular arrangement and the gradual differentiation of their 

 functions. 



Similarly, Godlewski's investigation fails to shed light upon the 

 problem of heredity itself. It shows us, on the other hand, quite clearly 

 and definitely that it is not right to regard the process of cell-division as a 

 function of the nucleus alone, and consider that the protoplasm plays a 

 passive part. The cell of the egg, in its entirety and according to its chem- 

 ical composition, must have, more than any other cells of the body, the 

 ability to subdivide and increase. The entire nature of cell propagation, 

 in all its particulars, has been found to be bound up with it. 



Loeb's experiments have shown that the egg-cells of various species of 

 animals may be easily induced to take part in such a process of cell-division. 

 The cells of one kind develop up to a certain stage apparently of their own 

 accord, others require a slight stimulation, while others need quite a con- 

 siderable impulse. The entire development of the individual is bound up 

 with countless problems. When we remember that we have before us a 

 mode of development which has been inherited for ages and has been 

 taking place over and over again in paths which have been well defined, 

 the wonderful thing is that there is apparently no limitation to it. We 

 can understand perhaps how, in the case of the mature organism, lost 

 tissues may be regenerated; and, again, it does not appear so remarkable 

 to us that from the cells of a certain tissue others may be produced with 

 the same function and the same chemical composition, so that the whole 

 corresponds to a morphological and functional unit. On the basis of our 

 present knowledge, however, it is a mystery how all the different tissues 

 can develop from a single cell, and how in each kind of tissue chemical 

 processes will take place which are peculiar to that particular tissue, as is 

 evident from a study of the secretions and of the end-products of their 

 metabolism. The complicated picture of the morphological development 

 of the individual seems to us far less remarkable than the much more 

 intricate and more involved differentiation of the chemical construction 

 and the chemical processes which take place in the individual cells. 



Now we know that the individual, in the case of the more highly organ- 

 ized animals especially, shows not only a development of species, but in 

 its beginnings there are stages of development to be detected which we 

 can understand only from the history of the ancestry of the animal organ- 

 ism. We need recall merely the formation of the gill-clefts or gill-slits, a 

 stage of development which even the human foetus passes through. Is it 

 not probable that there are differences in the chemical composition of the 

 tissues during the separate stages of its being, which are likewise suited 

 for tracing the entire group of vertebrates back to a common ground 

 plan? We must thank G. von Bunge for an answer to this question. 1 



1 Z. physiol. Chem. 28, 452 (1899). 



