128 IVAN E. WALLIN 



is still (lisagreenunit on the purely epithelial origin of the reticu- 

 lum and ITassal's ('()r])usclos, and the cardinal ])rohleni, the source 

 and nature of the de\'el()]nnent of the small thymus cells, has not 

 been definitely settled up to the present time. Three fundamen- 

 tally different views, each with its coterie of supporters, are held 

 rej2;ar(lino; the source of these cells. A large number of investi- 

 gators lielieve that the small thymus cells are true lymphocytes 

 which are formed from the epithelium by a process of trans- 

 formation. An equally large number believe that these cells 

 represent true lymphocytes which have wandered into the epi- 

 thelial anlage from the mesoderm. A remaining smaller group 

 of investigators believe that the small thymus cells have an 

 epithelial origin and are different from true lymphocytes. 



Even in the most primitive animals in which the thymus has 

 been established, the elasmobranchs, the formation of the small 

 thymus cells does not occur until mesodermal tissue is present 

 in the epithelial anlage. The source of a new type of cell which 

 forms in a mixed tissue would not l)e difficult to determine if the 

 two tissues entering into the formation had different morpho- 

 logical characters and retained them. The methods employed 

 up to the present time have not shown sufficient morphological 

 differences in the mesodermal and endodermal cells present in 

 the thymus anlage to establish the source of the small thymus 

 cells. 



The sudden appearance of the thymus as a well defined struc- 

 ture in the elasmobranchs, together with the probability of find- 

 ing a solution to the question of the source of the small thymus 

 cells, has stimulated a number of investigators to search for a 

 homologous structure in the more primitive types of chordate 

 animals. While the search has been a fruitless one in the asci- 

 dians and amphioxus, various structures have been described 

 for a thymus in the cyclostomes. The evidence offered in these 

 descriptions has not been sufficient to establish the organ in this 

 group of animals. The failure to find the thymus or its homo- 

 logue in the cyclostomes especially in the Petromyzontes, may 

 be attributed largely to the peculiar nature of the branchial 

 region in this primitive group of animals. 



