The origin and histogenesis of the thymus in Eaja batis. 427 



Figs. 14, 37, and 38 pass through the first, second, and fourth 

 placodes respectively. The sections figured lie near the dorsal ends 

 of {the placodes, and, whilst in the two former leucocytes are fairly 

 abundant, and in addition there are cells taking on leucocytic char- 

 acters, in the figure through the fourth placode the latter is seen to 

 be merely epithelial and to contain as yet very few leucocytes. 



Of the figures from embryo No. 203 Fig. 16 is from the first 

 placode. Figs. 39 and 42 from the third. The first two figures, Figs. 16 

 and 39, closely resemble figures from the previous embryo, and, there- 

 fore, they call for no detailed consideration. Fig. 42 shows other 

 things besides the thymus-placode, and these must now be described. 

 The limits of the thymus-placode are fairly easily determined, and at 

 one point it is seen to be sharply marked ofi' from a similar piece of 

 modified epithelium, abutting on it. This is a portion of a sensory 

 placode in connection with the vagus, and the figure serves to illus- 

 trate how, as elsewhere already stated (Beard, '00, II, p. 558), at 

 certain periods the sensory placodes come into rather close relation- 

 ships with the thymus ones. As this figure demonstrates, and as 

 appears in the actual preparations, the resemblances between the two 

 pieces of epithehum are very striking. Their epithelial cells are very 

 like and similarly arranged, and in both instances the conversion of 

 epithelial cells into other elements can be witnessed: in the one case 

 into leucocytes, in the other into nerve-cells or ganglion-cells, to use 

 Apathy's terms. 



Before leaving this set of embryos it may be pointed out, that 

 from the comparison of the figures of transverse and frontal sections 

 it is clear, that at this period a thymus-placode is a somewhat oval 

 plate of cells, depressed towards the centre, and with its longitudinal 

 axis about at right angles to that of the body. 



The next embryo to be considered is a very remarkable one in 

 the pictures it yields of the placodes. It is R. hafis No. 443. Un- 

 fortunately the embryo was not measured, or at any rate I can find 

 no record of its dimensions in any of my note-books or catalogues. 

 It is estimated to be about 25—27 mm. Sometimes I have been in- 

 clined to believe this to be an embryo of Raja radiata from the con- 

 ditions revealed in its thymus-placodes, but this idea has not found 

 confirmation in other points in its anatomy, and I hold it to be im- 

 possible, that in some way or other under this number a R. radiata 

 embryo can have been sectioned instead of a R. batis one. 



