362 



ou this plate becomes invagioated, forming a conical structure with 

 a central lumen, as in embryo no. 255, In others of the later period 

 of the development there is no cone, but instead thereof a thick 

 knob-like structure, resembling an ordinary thymus-element of Raja. 



The diiference is interesting, because it corresponds to a like 

 divergence, witnessed in the development of the ordinary thymus- 

 elements of Raja and of Scyllium. Time has been devoted to the 

 investigation of the thymus of Scyllium without any really adequate 

 reward in the way of results. Though my material of Scyllium is 

 sufficiently abundant, embracing some hundreds of embryos, that part 

 of it as yet studied was not preserved and stained in such ways as 

 to be very favourable for an investigation of the thymus. This organ 

 has been carefully examined in embryos of Scyllium of all phases up 

 to those of 39 mm. From this study one point has revealed itself 

 with great clearness, and this, rather than the methods of preservation 

 and of staining, perhaps accounts for the indifferent success. 



Though the thymus-elements of Scyllium arise after the same 

 fashion as those of Raja, for a long time their placodes remain very 

 simple, only increasing in area. This increase in area and the pro- 

 longed absence of leucocytes in the epithelium render it a task of 

 difficulty to define the limits of the latter — a point, in which Raja 

 never presents the least trouble. 



Later on the simple plates become bent inwards or invaginated, 

 and after this they present the appearances of long tubes, projecting 

 into the mesoderm. So far as my observations extend, the commence- 

 ment of the formation of leucocytes does not long precede the attain- 

 ment of this condition. 



On the formation of these tubes the growth of ordinary covering 

 epithelium over the base, as over a wound, takes place. It is due to 

 the circumstance, that Dohrn's observations commenced after the 

 formation of this covering epithelium, that he speaks of the more 

 superficial portion of the pouch-epithelium as taking no share in the 

 proliferation. 



He writes: "An diesen knospenförmigen Wucherungen nimmt nur 

 die innere Schicht des Epithels Anteil, die äußere zieht in dünner 

 Lage darüber weg" ^), From my work on Raja this statement was 

 challenged by me. As it now turns out, the criticism was based in 



1) A, DoHRN, Studien etc., No. 4, in Mitteil. a. d. Zool. Stat, zu 

 Neapel, 1884, p. 42. 



