ORGAN (TISSUE) DIFFERENTIALS 531 



some way connected with each other, differences in organismal differentials, 

 which distinguish individuals or species, being associated in a graded way with 

 differences in organ differentials and, in general, with differences in the mosaic 

 structure of the organism of which these organ differentials form a part. In 

 the embryo, the organismal differentials, as well as the organ differentials, are 

 not yet fully specialized ; in both cases only the more fundamental differentials, 

 the precursor substances of the finer differentials, exist. Thus the organ dif- 

 ferentials at a certain embryonal stage may be represented by the differentials 

 of the germ layers. Furthermore, the organizer (inductor or evocator) 

 substances in their origin are intimately connected with the mosaic characters 

 of the individual ; they are produced in differentiating organs or tissues and 

 initiate further differentiation in other organs or tissues. The organ differ- 

 entials represent, therefore, the specific structures and functions of the 

 tissues and organs within an organism ; they are intraorganismal differentials. 

 As we have stated previously, in this strict sense the organ differentials 

 represent the specific factors which are common to the same types of organs 

 in individuals belonging to the same species and in species belonging to the 

 same order or class; but in making these distinctions between homologous 

 organs and tissues, it must be understood that finer differences exist between 

 tissues in the same organism than is usually assumed. When we differentiate 

 between certain organs and tissues in an organism we use for this purpose, 

 as a rule, very obvious anatomical and histological peculiarities. Thus we 

 distinguish between organs, such as liver and kidney, and between tissues, 

 such as epithelium and connective tissue; among the epithelial tissues we 

 distinguish still further between stratified and glandular epithelium, and 

 among the connective tissues, between the ordinary collagenous and the 

 cartilage and bone-forming connective tissues. However, further analysis 

 shows that these are rather crude distinctions, that in reality much finer ones 

 exist between different tissues, which, on the basis of morphological criteria, 

 we are accustumed to consider as possessing essentially the same character. 

 As we have mentioned on previous occasions, biochemically the connective 

 tissue of the uterine mucosa differs from that of the Fallopian tube and 

 vagina, and even within the different parts of the cervix there are graded 

 differences in the connective tissue, as is shown by the response of these 

 tissues to the lutein and follicular hormones. Fibroblasts differ biochemically 

 in different areas of the embryo as to the amounts of acid they produce and 

 also in their resistance to acid and in their proliferative power. Furthermore, 

 Huggins has shown that the regenerating epithelium of the bladder has the 

 power to transform the fibroblasts of certain organs, but not those of others, 

 into osteoblasts, which are then able to produce bone. Apparently the 

 epithelium of the bladder gives off a contact substance, which acts as an 

 organizer and transforms only connective tissue cells of some organs into 

 osteoblasts. As these examples show, the differentiation of various tissues 

 is a much finer one than is assumed on the basis of ordinary morphological 

 criteria; but such finer differentiations have not yet been subjected to 

 serological tests and these would presumably be unsuccessful on account of 



