The origin and histogenesis of the thymus in Raja batis. 



413 



might easily be mistaken for the sensory placode of a gill-arch. Its 

 inner end is again sharply defined from the pharyngeal epithelium i), 

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

 epiblast and hypoblast, c. m, which formerly made up the outer boundary 

 of the pouch. 



A quite similar condition of affairs is met with in an embryo of 

 13 mm (No. 198). This is shown in text-figure D and in Fig. 20, 

 PI. 6. This phase only differs from the preceding one, in that the 

 placode has become turned somewhat more upwards and outwards. 

 The remains of the closing membrane are still in existence, and the 

 placode itself has undergone no apparent change. 



From this period onwards no remains of the closing membrane 

 have been seen. The absence of this and the completion of the 

 rotation of the placode through 90" lend it a different appearance 

 than it possessed in earlier phases. In embryos of 17 — 22 mm or 

 thereabouts there is little change in the shape or size of the placode, 

 and the conditions are generally represented for this period in text- 

 figure E and Figs. 5 and 19. 



These are from embryo No. 189. This probably measured in 

 alcohol about 21 mm, in the embedded condition its size was 20 mm. 

 It resembles two other embryos. Nos. 190 and 191. From No. 190, 



1) On p. 558 of the resume of this work (Beard, 1901, 2) in de- 

 scribing this the word "epiblast" has somehow slipped into the text 

 instead of "epithelium". 



