The origin and histogenesis of the thymus in Raja batis. 417 



or more leucocytes from peritoneal cells of a mammal i). But it is 

 not for a moment open to doubt, which of these two statements is 

 the more improbable. 



The main argument for the existence of more than one category 

 of leucocytes is the supposed specific nature of the "granula" in the 

 cytoplasm with various stains, the "granula" of different leucocytes 

 staining differently and specifically with certain mixed stains. Hesse's 

 inquiries into this question will be found in a recent number of the 

 Anat. Anz., and further reference may be made to this. The results 

 of his investigation go to disprove the existence of this supposed 

 specific character, and, thus, as it would seem, they tend to support 

 the views of Gulland. 



From the embryological standpoint little time need be wasted in 

 a useless discussion of the matter. In embryo-skate, even in such 

 late phases that one can properly no longer speak of embryos, there 

 is only one form of leucocyte. Moreover, the right must be denied 

 to pathologists and histologists to set up what are morphological kinds 

 of cells, whose history is not based upon the results of embryological 

 investigation. To recognise in morphology and embryology various 

 forms of leucocytes as distinct entities, like ganglion-cells and muscle- 

 cells, from the results of the study of diseased animals would be to 

 go back to the standpoint of more than a hundred years ago, when 

 among other things Cyclopean monsters were supposed to represent 

 former stages in the (ancestral) history of man. Even the histological 

 recognition is not sufficient to establish the specific character of a 

 certain kind of cell with certainty: this can only be done by develop- 

 mental research, by a study of histogenesis. And, be it added, the 

 vertebrate body is not big enough, and not sufficiently complicated in 

 structure, to admit of the existence of a number of different organs, 

 each of which should have as its function the formation of a different 

 sort of leucocyte. The conversion of the tissue-cells of other organs, 

 such as those of the peritoneum, into leucocytes is to the embryologist 

 not worthy of serious consideration. 



Following M. Heidenhain, Gulland, and others, therefore, in 

 the following lines it will be taken as certain, that in a vertebrate 



1) One or more observations of this kind were, I am told, stated 

 in refutation of the writer's conclusion, that the thymus was the parent- 

 source of all the leucocytes of the body, by a pathologist of high 

 standing. I have not been able to find any published observations of 

 this kind. 



Zool. Jahrb. XVII. Abth. f. Morph. 27 



