THE CAUSATION OF DISEASE. 45 



special and peculiar properties possessed by these cells. It seems 

 obvious that this mysterious power which they display, of originating 

 a new and complex organism, distinguishes them in the broadest way 

 from portions of organic substance in general. Nevertheless, the 

 more we study the evidence, the more is the assumption shaken — the 

 more are we led towards the conclusion, that these cells have not been 

 made by some unusual elaboration, fundamentally different from all 

 other cells. The iirst fact which points to this conclusion, is the fact 

 recently dwelt upon, that in many plants and inferior animals, a small 

 fragment of tissue that is but little differentiated, is capable of deve- 

 loping into the form of the organism from which it was taken. Con- 

 clusive proof obliged us to admit, that the component units of 

 organisms, have inherent powers of arranging themselves into the 

 forms of the organism to which they belong. And if to these com- 

 ponent units, .... such powers must be conceded — if, under fit 

 conditions, and when not much specialized, they manifest such powers 

 in a way as marked as that in which the contents of sperm-cells and 

 germ-cells manifest them ; then, it becomes clear that the properties 

 of sperm-cells are not so peculiar as we are apt to assume. Again r 

 the organs for preparing sperm-cells and germ-cells, have none of the 

 speciality of structure which might be looked for, did sperm-cells and 

 germ-cells need endowing with properties essentially unlike those of 

 other organic agents. On the contrary, these reproductive centres 

 proceed from tissues that are characterized by their low organization. ' r 



He then proceeds to exemplify this statement in the case of 

 plants and animals : — 



" The embryo cells are formed in the undifferentiated part of the 

 cambium-layer; the pollen grains are formed at the little-differen- 

 tiated extremities of the stamens ; and both are homologous with 

 simple epithelial-cells. Among many inferior animals devoid of special 

 reproductive organs, such as the Hydra, the ova and spermatozoa 

 originate in the layer of indifferent tissue that lies between the 

 endoderm and ectoderm ; that is, they consist of portions of the least 

 specialized substance. And in the higher animals these same 

 generative agents appear to be merely modified epithelial cells — cells 

 not remarkable for their complexity of structure, but rather for their 

 simplicity." 



He next remarks that undifferentiated epithelial cells are often 

 observed to exhibit the like property. Thus, referring to a cer- 

 tain plant, he learns from Dr. Hooker that " many young plants 



