INDIVIDUAL DEVELOPMENT 243 



and, not only so, but, in bilateral animals, it is bilaterally symmetrical. 

 This is not usually indicated in the form of the ovum, which is typically 

 spherical, but in the disposition and developmental value of its parts. 

 Here we have one of the most fundamental and least comprehended 

 facts in embryology. It has, moreover, been shown that this property 

 of direction and localization resides in the homogeneous, transparent, 

 semifluid matrix that suspends all the visible particles of the proto- 

 plasm of the egg. It is probable that primordia of all grades possess 

 similar properties, and, if this is so, we have a principle that goes far 

 to explain the orderly localization of processes in morphogenesis. 



This principle is not farther analyzable at present ; but, as it may be 

 found intact in parts of primordia no less than in the whole, it prob- 

 ably rests on a molecular basis. The most ready analogy in simpler 

 phenomena is that of crystallization. The study of fluid crystals has 

 furnished us examples of inorganic molecular aggregates in which 

 direction and localization are given in the whole and also reappear 

 rapidly in the parts when the whole is subdivided. 



3. The Role of Cell-division in Development. — The individual or- 

 ganism begins as a single cell, from which all cells of the developed 

 organism trace their lineage by the process of cell-division. This has 

 been regarded as one of the most fundamental factors of the individual 

 development in the theories of Weismann, Hertwig and others. But 

 important as the process of cell-division undoubtably is in development, 

 I believe that it is impossible to ascribe to it in principle more than an 

 indirect effect: Considerable complexity of development is possible 

 among Protozoa, whose body is unicellular, and some ova may carry 

 out under experimental conditions a considerable part of the early 

 development without a single cell-division. Moreover, the same kind 

 of differentiated structure may be composed of one cell or of many, 

 or of variable numbers of cells. 



The physiological value of cell-division is no different in principle 

 in developing than in functioning tissues (using these terms in the 

 usual sense). The general law of relative reduction of surface in 

 proportion to increasing mass imposes a size limit on cells, which can 

 be regulated only by cell-division; an internal principle of regulation 

 of cell-size has also been stated by E. Hertwig and Boveri, viz., a cer- 

 tain relationship characteristic of each species between the amounts 

 of nuclear and cytoplasmic matters, so that increase of initial volume 

 of the former involves increase of the latter, and vice versa. Corre- 

 sponding to these principles, we find that individuals of different sizes 

 of the same species vary not in the size, but in the number of the cells ; 

 and this is regulated by variation in the number of cell-divisions in 

 different individuals. 



Cell-division must necessarily, therefore, have an immense func- 



