Franklin P. ,Mall 147 



own and continued a forward course. He now became the leading advo- 

 cate of the theory of mechanics in development and it subsequently 

 tinctured all of his papers. Although he made no experiments upon the 

 growth of animals, he must be viewed as one of the pioneers of the new, 

 science of experimental morphology. 



Throughout His's embryological papers we see that he believed that 

 the form of the animal body, as well as of its organs and tissues, is due 

 to mechanical influences of the structures upon one another. This con- 

 ception was often erroneously construed as meaning that there is a me- 

 chanical cause for growth, but His repeatedly denied this. " The stimu- 

 lant which causes cells to multiply cannot be traced to mechanical 

 influences." His mechanical conceptions of development are compara- 

 ble with the study of the mechanics of the circulation rather than with 

 that of the evolution of the heart. When analyzed, it appears to me 

 that his mechanical conceptions are related principally to the wandering 

 of tissues and organs in development. One of the best examples of His's 

 ideas is his conception of the growth of nerve fiber from the central cell 

 to the periphery, where, through secondary connections, it makes itself 

 fast to its end organ. With this conception, we can understand and 

 picture to ourselves the formation and the infinite number of variations 

 of the peripheral nervous system. Other examples may be found in the 

 wandering of the diaphragm and in the metamorphosis of the branchial 

 arches. Mechanical influences must guide these structures to their fate. 

 And finally, great masses of tissue wander in the embryo long before we 

 can see what is to become of them. The best example of this kind is to 

 be found in the early development of fishes formulated in His's brilliant 

 theory of concrescence. According to His, the word "mechanical" is 

 to be applied to the movements of these avalanches of tissues and their 

 influence upon one another in gaining their final position and not to the 

 cause of growth. 



One of the characteristics of His's embryological work is that he viewed 

 the embryo as a whole ; he always approved of embryological papers which 

 carried a subject to its logical conclusion.'" As he improved the micro- 

 tome more and more, so that he could cut serial sections fifty microns 

 thick, he constantly kept in mind the relation of the individual section 

 to the embryo as a whole; this led to his well-known method of graphic 

 reconstruction in 1868. At that time haphazard sections of an embryo 

 were often compared with chance sections of older embryos which led to 

 all kinds of erroneous conclusions. In the course of time, he perfected 

 his method by drawing an enlarged picture of the embryo upon ruled 

 paper into which he projected in their proper positions the sections 



