410 Nose, Phar3'2ix and its D.erivatives in Man 



ments arise just at the junction between the " epiblast and hypoblast," 

 and he suggests that some cells of both layers are included in the rudi- 

 ments and that the two kinds have different fates; that in the adult 

 the corpuscles of Hassal represent the remains of the epiblastic cells, 

 while the lymphoid tissue is the transformation hypoblastic cells. From 

 its point of origin he compares its function to that of the tonsil and 

 thinks that it is an organ developed to guard the gills and pharyngeal 

 region. Capobianco '* believes the lymphoid tissues are cells which 

 have immigrated into the primary rudiment and that the l5^mphatic 

 structure of the thymus was acquired secondarily. Kastchenko," who 

 worked on pig embryos, described the various details of the develop- 

 ment of the thymus with great care. He thought the greater part of 

 it came from the epithelium of the 3rd visceral pouch but that part 

 also came from the sinus precervicalis. 



Harman ^° describes the condition found in two babies at term where 

 there were a socia thymi cervicalis and a thymus accessorius present, 

 He refers to the condition in the sheep, where he says the 3rd and 4th 

 visceral pouches participate in the formation of the thymus, and sug- 

 gests that where there is an accessory thymus in man there may have 

 been a reversion and the 4th pouch had given rise to it. He quotes 

 Sir Astley Cooper as saying that he has frequently observed that the 

 cervical portion of the thymus is higher on the right than on the 

 left side. 



In the present study the model of embryo II shows the 3rd visceral 

 pouch as a ridge with a ventral free end (V. P.'", Fig. 2), but no 

 differentiation of tissue to suggest a thymus. In the model of embryo 

 CLXIII the prominent ridge has disappeared and the 3rd visceral pouch 

 projects out directly from the pharynx. This pouch has a slightly 

 enlarged end (V. P." in Figs. 4 and 5). In the next stage, in an 

 embryo of about 4^ weeks of age, the thymus (Thym., Fig. 7) approaches 

 very closely the condition figured by Born,'^ on page 297 (fig. d.), in 

 which the thymus has completely lost its connection with the phar3^nx, 

 and its original hollow is closed to a crescent-shaped opening that is 



18 Capobianco: Contribution a la morphol. du thj'mus. Arch. ital. de biol., XVII, 

 1892. 



IS Kastchenko: Das Schlundspalteusystem des Huhncliens. Arch. f. Anat. u. Pliys., 

 Anat. Abth., 1887. 



2" N. Bishop Harman: "Socia thymi cervicalis" and Thymus accessorius. The 

 Journal of Anatomy and Physiology, vol. XXXVI, Ft. 1, 1901. 



"1 Ueber die Derivate der embryonalen Schlundbogen. Archiv f. mikros. Anat., Bd. 

 XXII, 1882. 



