INDIVIDUAL DEVELOPMENT 241 



of the germ-plasm, includes the assumptions of the pangenesis hypoth- 

 esis, with those eliminated that were made necessary by the conception 

 of the inheritance of acquired characters. For Weismann's gemmules, 

 or determinants, the assumption of somatic origin was unnecessary, 

 and thus, as Professor Whitman states, the entire centripetal migration 

 of Darwin's theory was eliminated, but the entire centrifugal process 

 was retained. The origin of every character of the individual was 

 explained in the Weismannian theory, as in the Darwinian theory, by 

 the unfolding (it can not be called development) of representative 

 particles. Nevertheless, the theory of the germ-plasm played an im- 

 portant role in the development of biological knowledge, for it framed 

 a set of ideas in a manner sufficiently logical and definite to serve as 

 veritable working hypotheses or bases of attack. The immense effect 

 of "Weismann's writings on the theory of individual development should 

 not be underestimated. 



Physiology of Development 



The theories of individual development that we have mentioned 

 bear all the marks of provisional or formal hypotheses. Although 

 extremely ingenious and logical, they are based only in small part on 

 analysis of the actual processes and they offer no real explanation of the 

 phenomena themselves; for they really include all the elemental phe- 

 nomena and merely sum them up ; they are definitions that include the 

 matter to be defined ; they amount to a denial of the reality of individ- 

 ual development as truly as did the preformation theories of the 

 eighteenth century. 



As a series of processes occurring in nature and accessible to experi- 

 ence, the development of the individual is capable of resolution into 

 simpler biological processes, and these presumably into physico-chemical 

 events in the usual sense. All attempts to make such analyses come 

 under the head of Physiology of Development; and this plan of 

 attack on the problems of individual development, known in Germany 

 as developmental mechanics, is one of the most actively pursued lines 

 of biological investigation at the present time. Physiology of Develop- 

 ment deals primarily with specific problems, and the results constitute 

 a critical basis for the appreciation of general theories of both indi- 

 vidual and racial development. We shall examine some results and 

 principles of these studies, and consider their application to some 

 theories of heredity and evolution. 



1. Embryonic Primordia and the Law of Genetic Restriction. — 

 In the course of development the most general features of organization 

 arise first, and those that are successively less general in the order of 

 their specialization. Thus the directions of symmetry of the future 

 organism — the oral and aboral surfaces, right and left sides, anterior 



VOL. LXXV. — 16. 



