J 



The origin aud histogenesis of the thymus in Raja batis. 439 



of a real thymus-element could be encountered. And this remained 

 the position of affairs, until some time after all the chief points of 

 the present work had been established. Thus it came about, that 

 only by the aid of the facts of the development of an ordinary thymus- 

 placode into a thymus-element was the true history of the spiracular 

 thymus to be cleared up. 



In the last of the plates there are six figures (Figs. 61 — 66) of 

 the spiracular thymus from as many different embryos of various 

 periods. It would have been easy to have greatly increased this 

 number, or to have worked out a monograph upon the spiracular 

 thymus alone. The six figures are typical of the chief points en- 

 countered in its history, and they may be said to establish beyond 

 doubt the morphological nature of the structure in question. 



In the original note (1900, I) upon the spiracular thymus it was 

 stated, that the earliest embryo, in which in the placode of the hyoid 

 cleft leucocytes had been seen, was 28 mm in length. Since then 

 they have been found in others of similar dimensions. The early 

 history of the spiracular placode is exactly like that of any other of 

 the placodes. The differences between it and them being that it for 

 a long period only increases in extent or area, and that leucocytes 

 are formed within it only at a late period, or, if they arise earlier, 

 their number is so small as to enable them to elude observation. In 

 the latter respect there is some resemblance between this placode and 

 that of the last cleft. 



As to the precise epoch, at which leucocytes appear in the spiracular 

 placode, no definite statement can be made. All that can be said is, 

 that to all appearance no leucocytes are formed within its epithelium 

 in embryos earlier than those of 25 mm. Owing to its rudimentary 

 character it is, perhaps, not unlikely that sometimes they may only 

 appear much later. 



As is well known , the spiracular cleft attains much larger 

 dimensions than the true branchial clefts during a period of the 

 early development. And later on it undergoes a considerable reduction. 

 Whatever else be concerned in this increase in size, the placode cer- 

 tainly shares in it. It attains a very considerable area, even in 

 embryos of 25 mm. For this reason in Fig. 61 from embryo No. 443 

 only a portion of it has been drawn. As this figure shows, it is made 

 up of one layer of columnar epithelial cells, and among these one or 

 two leucocytes are seen. Abutting upon the dorsal edge of the placode 

 is the sensory placode of the hyomandibular nerve. This latter is 



