558 



the epithelium is as well marked otf from the much flatter epithelium 

 of the pharyngeal hypoblast. 



The next embryo to be noted (Xo. 163) is about 10 ram in length. 

 There are 101 somites present. The spiracle and the first two bran- 

 chial clefts are open, and the remaining three are represented by 

 pouches. The same piece of modified epithelium or placode is still 

 obvious just above the opening of the first branchial cleft. It is now 

 turned upwards and outwards, in such a way that, as compared with 

 its former position before the cleft was open, its upper and dorsal end 

 has been raised through an angle of about 45°. It has thus acquired 

 a position, which makes it appear to be a portion of the epiblast. 



Its inner end is again sharply defined from the pharyngeal epi- 

 blast, and its outer end abuts against the remains of the membrane 

 of epiblast and hypoblast, which formerly made up the outer boundary 

 of the pouch. A quite similar condition is met with in embryos of 

 13 mm and 17 mm, except that remains of the membrane no longer 

 exist. In embryos of 19 to 20 mm the whole piece of epithelium is 

 as well-defined as ever, but it has wandered or been pushed upwards 

 and outwards by growth of the hypoblast. It now lies on the level 

 of the notochord, and the plane of its surface is almost at right angles 

 to what was this plane during the gill-pouch-period. Any idea that 

 this placode is, or has any connection with, the neuroepithelium of a 

 branchial or lateral sense organ must be distinctly repudiated. There 

 is, perhaps, no investigator to whose eyes such a piece of neuro- 

 epithelium is more familiar. The latter was originally described by 

 me some fifteen years ago , and I have neither forgotten my own 

 work, nor neglected the neuroepithelia of the branchial or lateral sense 

 organs since then. That the neuroepithelia of the latter do in certain 

 stages come into rather close contact with the placodes of the thymus 

 in both Raja and Scy Ilium is certain, and more attention would 

 be devoted to the matter in my complete memoir. 



The position and form of this modified piece of epithelium or 

 placode remain much the same until the embryo is 25 mm in length. 

 By this time numerous leucocytes have formed within the epithelium, 

 and many of them have already wandered out, as will presently be 

 described. 



As revealed by its subsequent history, this modified piece of 

 epithelium or placode is the rudiment or foundation of a thymus- 

 element. 



Thus, in the skate the thymus-elements, five on each side, arise 

 as specialised portions of the dorsal epithelium of the respective gill- 



