PHYSIOLOGY OF CYTOST ACTION 83 



ing structure endowed with facilities for locomotion, 

 while the nervous system serves mainly as a coordi- 

 nating mechanism. The latter tissues are, of course, 

 dependent upon the normal functioning of the viscera 

 for their continued existence. 



What have the various viscera in common that can 

 account for their ready susceptibility to the action of 

 cytost? While this question cannot be completely an- 

 swered, a valuable suggestion is found in their com- 

 mon histogenesis. As the reader is aware, in the early 

 stages of embryonic development three distinct germ 

 layers are formed. These are termed respectively the 

 ectoderm, the mesoderm, and the endoderm (ento- 

 derm). 



Embryologists have been able to follow the subse- 

 quent development of these primitive layers into the 

 various tissues of the body. During the course of 

 embryonic development, the innermost layer, or en- 

 doderm of the vertebrate, has been found largely to 

 retain its original epithelial character. From it are 

 formed the epithelial lining of the enteron, its auxil- 

 iary organs and glands — the stomach, liver, pancreas, 

 lungs, and urinary bladder. 



Those organs which are particularly sensitive to 

 cytost have then a common origin, and it is perhaps 

 justifiable to conclude that their peculiar sensitivity 

 is simply that of a primitive germ layer, the endo- 

 derm. This conclusion seems further justified when 

 it is remembered that of the three germ layers, the 

 endoderm suffers the least differentiation during de- 

 velopment. 



The marked sensitiveness of the tissues of endo- 

 dermal origin was first stressed by the author in con- 



