222 Henry Fox 



groove and thus occupies the upper part of the pericardial region. It 

 retains its tubular form, but shows a differentiation into two portions — 

 a terminal, swollen portion and an intermediate, relatively narrow canal, 

 which connects the former with the remaining dorsal body of the pouch. 

 The latter now forms a compressed epithelial plate, bearing upon its 

 anterior surface the voluminous carotid gland. It is still connected 

 with the pharynx by a narrow connective. 



The dorsal apex of the pouch has disappeared. The peripheral por- 

 tion of the pouch is attached for a short distance to the anterior wall 

 of the fundus prsecervicalis (F.Pc). 



A lumen is present in the tubular downgrowth and in the pharyngeal 

 connective, but has disappeared into the dorso-peripheral part by approxi- 

 mation of its anterior and posterior walls. The latter thus assumes the 

 form of a vertical plate, connected with the fundus prsecervicalis at its 

 dorso-lateral angle and continuous with the intermediate cervical canal 

 of the thymus at its ventro-internal angle. 



In the 17 mm. pig (No. 51, Harvard collection. Figs. 19-21) the 

 thymus downgrowth (Thy.) has still further elongated and now shows 

 clearly its segmentation into three portions. These include (1) the 

 considerably swollen thoracic vesicle, (3) the intermediate cervical con- 

 nective (CV.C), and (3) the dorsal plate (D.Pl.), with which is closely 

 associated the carotid gland (C.Gl.) and fundus prscervicalis (F.Pc). 



Only a few fragments of the original lumen now remain, mostly 

 confined to the thoracic vesicle. 



Connection with the pharynx is still maintained by an extremely 

 thin, solid connective (Fig. 45, of an 18 mm. pig.). 



The dorsal body of the pouch, as already noticed, has become reduced 

 to a flattened plate and now constitutes a part of thymus. It has 

 separated from the ectoderm of the surface of the body, but remains 

 attached to that of the fundus prascervicalis, which constricts from the 

 superficial ectoderm and accompanies the thymus as the latter passively 

 recedes from the exterior. The detailed account of this process will be 

 reserved for later treatment. It suffices at this time to state that this 

 ectodermal structure remains in close connection with the dorsal ex- 

 tremity of the thymus for a considerable period. 



In an 18 mm. pig the thymus shows no essential deviations from that 

 in the preceding stage. It is slightly longer and has a more vertical 

 course than in the latter. 



