The origin and histogenesis of the thymus in Raja batis. 433 



It would unduly increase the size of this memoir to give an ac- 

 count of the emigration of leucocytes from the placodes of all the 

 specimens, for the results are based upon the study of more than 

 twenty embryos of sizes from 28 to 42 mm. In almost every section 

 through every placode of all the embryos there are breaks, and it is 

 quite out of question to suppose for a moment, that they are artefacts. 

 All the breaks were in existence before the sections of any given 

 embryo were made, for all the leucocytes, like other structures in the 

 embryo, are firmly fixed to the glass by the albumen employed. And, 

 indeed, I have yet to see a skate-embryo of 28 — 41 mm, in which 

 such breaks in the anterior placodes are wanting. Considering the 

 number of sections passing through each of the placodes, the mere 

 recital of all the breaks of, say, but a dozen embryos would itself 

 occupy much space. 



But the evidences of the emigration of leucocytes from the thymus- 

 placodes of Raja^ as afforded by my sections, are quite overwhelming. 

 And, since opposed to the facts here recorded there are but the hypo- 

 theses of more than twenty years ago, which, moreover, have never 

 obtained any basis of fact in their support in the interval, for these 

 reasons the facts and figures here given may suffice. 



VIII. The later History of the Thymus-Placodes. 



Embryos of 37 — 70 mm. 



Of this period of the development, culminating in young skate of 

 7 cm, with one exception (Fig. 55) there are no figures in the plates. 

 The placodes of earlier embryos no longer deserve to retain this name, 

 for they gradually become thicker and thicker, and, finally, in embryos 

 of 42 mm onwards they project inwards as long pear-shaped struc- 

 tures. As DoHRN (1884) has already noted, of the 5 thymus-elements 

 of each side in Raja the 4 anterior ones attain respectable dimen- 

 sions, the 5. remains small. This is best observed in longitudinal 

 vertical sections, thus, in jR. hatis No, 252 (58 mm) of the 5 elements 

 the 1. and 2, are large, the 3. and 4. smaller, while the 5, is very 

 small. Although this is the case with the last element, it is also in 

 fact a miniature edition of any of the other 4, and one can follow in 

 it all the changes, through which the others pass. 



For this reason, and because of its minuteness, it had been in- 

 tended to have illustrated the later history of the thymus-elements 



Zool. Jahrb. XVII. Abth. f. Morph. 28 



