The origin and histogenesis of the thymus in Raja batis. 443 



animal, in which the greatest number of thymus-elements has been 

 recorded ^). Moreover, it throws, I think, new light upon various 

 researches as to the number of thymus-elements in certain animals, 

 beginning with those of de Meuron (1886) and ending with the recent 

 ones of F. Livini (1902). There is no wish on my part to depreciate 

 these investigations in the least, but in one important point they lack 

 completeness. In none of them is it proved, that in connection with 

 the remaining gill-clefts or pouches at some period or other of the 

 early development rudimentary thymus-elements do not exist. There- 

 fore, these researches afford no information as to the maximum number 

 of thymus-elements, laid down in the particular forms, of which they 

 treat. What they reveal is in connection with which cleft or clefts 

 persisting well-developed thymus-elements are found. Apart from 

 these there may be thymus-placodes, which either never become 

 anything more than mere epithelial plates, or from which a few leuco- 

 cytes may be formed at some early period of the development, and 

 then the structure may disappear. It is quite conceivable, that, for 

 example, the spiracular placode might never get beyond the condition, 

 shown in embryos of 28 mm of Baja, in some other fish. 



In this way, and thus only, the sudden appearance of an acces- 

 sory thymus-element in some individual of one of the higher animals 

 becomes explicable. 



X. Thymus-Placodes and sensory Placodes. 



As pointed out in preceding pages, ' and as shown in several of 

 the figures, thus, in Figs. 42, 45, 61 and 62, the sensory placode in 

 its growth ventralwards enters at certain periods into close topo- 

 graphical relationships with the thymus-placode of the same arch. As 

 will presently appear, this fact was first noted, but wrongly inter- 

 preted, by Feoriep (1891). His mistake has reflected rather seriously 

 upon previous correct results of mine, and for this and certain other 

 reasons, to be subsequently developed, the present opportunity of dis- 

 cussing matters concerning the branchial or lateral sense organs is 

 welcome. 



Many years ago (1885) from its typical position, on the level of 

 the notochord and immediately above the branchial cleft, the original 

 foundation of a portion of the system of lateral sense organs, what 



1) With the exception of the lamprey, where according to Schaffer's 

 researches there would be seven pairs of thymus-elements. 



