The origin and histogenesis of the thymus in Raja batis. 429 



placodes of certain periods in R. radiata. Thus, in M. radiata No.' 230 

 (25 mm) the leucocytes in any one of the anterior placodes form a 

 big nest near its centre, and epithelial cells are restricted to the ends. 

 B. radiata No. 212 (26.75 mm) is quite similar, and very much the 

 same conditions obtain in R. radiata No. 204 (22 mm). Finally, a 

 section through a placode of a B. radiata embryo (No. 242) of 27 mm 

 is shown in Fig. 50. Here the entire interior of the placode — if 

 one may still speak of it as such — has been converted into leuco- 

 cytes, and of the original epithelial cells there is left merely a sur- 

 face-layer, forming a sort of shell around the whole. 



The final embryos of this epoch to be described are two of the 

 newer series of 1899. B. hatis No. 616 (26.5 mm) and No. 629 (27 mm) : 

 from the former the figures are Figs. 45 and 46, from the latter 

 Fig, 44. The comparison of these figures with those from embryo 

 No. 443 may be of interest to the reader. Figs. 45 and 46 represent 

 under different magnifications a section through the second left pla- 

 code, and Fig. 45 also shows adjacent structures. In this in addition 

 to the thymus-placode a piece of a sensory placode of the vagus, with 

 the connected nerve-twig, is seen in close connection with the thymus- 

 placode. This figure will be referred to again in another section, and 

 it may be compared with Fig. 42 already mentioned. It is intended 

 to illustrate how at a certain period of the development the sensory 

 placode becomes topographically closely associated with the thymus 

 one, and how the former in its growth pushes its way along the 

 latter. Fig. 46 reveals things similar to those already described in 

 previous embryos, and, therefore, its detailed description may not be 

 called for. 



Summing up the characteristics of the anterior thymus-placodes 

 during this period, they are: epithelial cells are still largely re- 

 presented, especially at the two ends of the placode, but also else- 

 where; many of the epithelial cells are taking on the characters of 

 leucocytes; many others have already done this; leucocytes are more 

 numerous in the placodes than in earlier embryos; their emigration 

 singly may still be readily observed, but there is an undeniable ten- 

 dency exhibited for the single passage to give place to a migration 

 from the placodes in numbers. As will be seen, the process here in 

 initiation is especially typical of the following epoch in the history of 

 the anterior placodes. 



