The origiu and histogenesis of the thymus in Raja batis. 



415 



up to young skate of 7 cm or more; but, as along with corresponding 

 figures of the plates they are only intended to prove ad oculos the 

 origin of the thymus-placodes, one more may suffice. This is text- 

 figure G from an embryo of 

 R. hatis, No. 209, (34 mm), 

 and the section employed 

 is also shown, so far as the 

 placode of the apparent left 

 side is concerned, in Fig. 17 

 PI. 5. The placode is now 

 much thicker and it has 

 begun to bulge inwards. Its 

 actual extension along the 

 lateral surface of the body 

 is, perhaps, not greater than 

 in embryos of 25 mm. As 

 will appear in the later ac- 

 count, most of its constituent 

 cells have now become con- 

 verted into leucocytes. At 

 a period like this it would 

 be impossible for the merest 

 tyro in embryology to mis- 

 take the thymus -placode 

 for anything else. 



In the skate, therefore, the thymus-elements, apart from the 

 rudimentary spiracular thymus, arise as specialised portions of the 

 dorsal epithelium of each and every true gill-pouch at a very early 

 period, before the pouch is open to the exterior. As will appear later, 

 the spiracular thymus has a quite similar origin, and, therefore, in 

 the skate in connection with the six pairs of gill-pouches there are as 

 many pairs of placodes of hypoblast of the dorsal walls of the pouches. 

 Without any doubt the thymus of Raja is a product of the hypoblast. 



As is well known, Kölliker (1879) was the first to maintain, that 

 in the rabbit the thymus arose from (the wall of) a modified gill-cleft, 

 but its hypoblastic nature and its origin as a modified portion of the 

 hypoblastic lining of a gill-pouch were not clearly determined. His 

 held, that in mammals the thymus was of epiblastic origin — a view 

 not supported by the researches of Kölliker, Gustav Born, and 

 ScHULTZE, and since rejected by His himself. Dohrn's (1884) and 



«^ -) 



G 



