SUBSTRATUM AND MORPHOGENIC SUBSTANCES 455 



substratum begin to predominate more and more over the environmental 

 stimulating factors, although some general conditions, such as the action of 

 more specific hormone-like substances and the state of nourishment and 

 oxygen supply, remain of importance. In regard to the latter factors, the 

 degree of differentiation may depend, in some instances, upon the influence 

 which relatively unfavorable conditions of nourishment exert on the tissues. 

 To cite an example : there can be little doubt that the keratinization of the 

 epidermis is due to the distance of epidermal cells, undergoing this change, 

 from the capillaries, a distance which increases in proportion to the intensity 

 of proliferation of these cells. The same factors may also affect the rapidity 

 and character of cell division ; a certain degree of unfavorableness in a 

 constellation of factors may act as a stimulus, initiating cell division, but at 

 the same time the unfavorable conditions may make the cell division thus 

 induced irregular. On the other hand, conditions which induce rapid normal 

 cell division may thereby inhibit a complete differentiation of the resulting 

 tissues. 



Experimental analysis of tissues by the use of hormones, as well as by 

 other means, has made it possible to establish the existence of a much greater 

 differentiation and individualization of , tissues in different areas than had 

 previously been assumed to exist, when observation of the structure of 

 tissues seemed to indicate their identity. Furthermore, the characteristics of 

 tissues and organs of an organism are determined by factors inherent in the 

 recipient tissues as well as by stimuli acting on them. While in different cases 

 the relative importance of these two sets of factors varies, in many instances 

 the former preponderate. This is true at least when the more differentiated, 

 phylogenetically and ontogenetically mature stages have been reached, while 

 in the more primitive stages the stimulating and transforming effects of 

 contact and distance hormone-like substances play a prominent role. 



There must be added to these morphogenic substances, certain vitamins 

 which also can be shown to have specific morphogenic effects under some 

 conditions ; this holds good, for instance, in the case of vitamin A, the 

 absence of which may produce a transformation of cylindrical into squamous 

 epithelium in some epithelial membranes ; also of vitamin D, which affects in 

 a specific manner the bony structures. With furthergoing phylogenetic and 

 ontogenetic development certain tissue differentiations take place, requiring 

 the presence of definite vitamins for the maintenance of normal structure 

 and function. 



The problem which we have discussed in this chapter is a part of the 

 wider problem as to the relative significance of living substratum and 

 environment in the development and function of living matter, whether of 

 individuals, species, or wider classes of organisms. The tissues which are the 

 bearers of the organismal and the organ and tissue differentials and their 

 precursors represent the substratum, and in this substratum the organismal 

 differentials and their precursors are the most constant constituents, while 

 the organ differentials seem to be more modifiable; the contact substances 

 and hormones represent a part of the inner environment, which however, can 



