482 



A natural enquiry is then that as to what light this may throw 

 on the functions of the thymus in fishes. 



Maurer describes the organ as underjoing an involution and 

 degeneration in adult life in fishes. This cannot be regarded as yet 

 proved on as probable. He failed to find it in a few cases, but Leydig i ) 

 speaks of the organ as though it were constantly present. In this con- 

 nection I feel obliged to quote from a previous work of Maurer's. 

 After describing the position in which, according to Leydig and Stan- 

 nius, the thymus of fishes was to be found, he says "Ein derartig 

 gebautes und gelagertes Organ konnte ich bei den von mir untersuchten 

 Fischen nicht nachweisen" 2 ). 



Personally I have not as yet examined the organ in any Teleost, 

 but among the great number of specimens of Raja which are annually 

 dissected in the practical classes of this University, I have never found 

 the slighest difficulty in recognising the presence of a thymus in any 

 specimen. 



The danger must be guarded against of expecting to meet with 

 identical conditions in the history of the thymus of fishes and of 

 Mammals. Differences in the initial stages of development have been 

 referred to above. 



If the organ be originally one in the service of the gills, one 

 would certainly anticipate changes in those forms in which gills were 

 no longer functional organs. In one very obvious respect this is 

 certainly the case. 



According to de Meuron 3 ) the thymus of the lizard arises from 

 three, that of the chick from one cleft. As great a reduction as in 

 the latter case is encountered in Mammals. Here too the organ is of 

 functional importance only during early life, and certainly for a much 

 shorter period of life than in fishes, even if Maurer were admitted 

 to be in the light as to its ultimate degeneration in the latter. 



Probable Function of the Thymus. 



The origin of leucocytes from the epithelium in the immediate 



neighbourhood of the gills can, I think, have only one meaning. They 



must be formed for the protection of the gills themselves. Possibly 



they serve for the devouring and removal of parts of the gills which 



1) Lehrbuch der Histologie, 1857, p. 431. 



2) F. Maueee, Morph. Jahrb., Bd. 9, p. 246. 



3) de Meuron, Eecherches sur le de'veloppement du thymus et de la 

 glande thyroi'de. Eeceuil Zool. Suisse, 1886, T. 3. 



