98 



Miss E. A. Fraser and Prof. J. P. Hill. 



form an elongated tubular structure and its closing membrane becomes 

 reduced in extent. At the same time, the portion of the ectodermal groove 

 situated immediately above the sinus separates off from the ectoderm in 

 continuity with the distal extremity of the second pouch. As the result, the 

 latter comes to be connected with the sinus by a short distal segment, the 

 ductus ecto-entobranchialis II above mentioned, formed partly of groove- 

 ectoderm, partly of pouch-entoderm, tbe line of junction of the two parts 

 being of the nature of an oblique overlap. The cervical sinus has meantime 

 closed, partly, and indeed mainly, as the result of the growth forward of its 

 dorso-caudal margins, partly as the result of the backgrowth of the hyoid 

 arch, its original wide opening becoming reduced to a narrow slit-like passage, 

 the cervical duct. 



The primordium of the cervical thymus first appears in the form of a 

 bulbous enlargement of the coalesced and thickened walls of the dorso-cranial 

 angle of the sinus, which passes above into direct continuity with the ductus 

 ecto-entobranchialis II, whilst below it extends down as far as the junction 

 of pouch 3 with the sinus-ectoderm. As development proceeds, the primor- 

 dium increases in size at the expense of the remainder of the cranial p6rtion 

 of the sinus, which completely closes up. It thus assumes the form of 

 a solid pear-shaped mass composed of epithelial cells. It is connected for a 

 time with the ectoderm of the cervical groove by a thin cellular cord but that 

 eventually disappears and the primordium lies free in the mesoderm. It is 

 thus evident that the epithelial basis of the cervical thymus is in greater part 

 of ectodermal origin, but it seems probable that a small amount of entoderm 

 derived from the ventral continuation of the second pouch is also included 

 in it. 



As concerns the differentiation of the third pouch, the connection of the- 

 latter with the pharynx gradually becomes narrowed, and at the same time 

 its connection with the sinus-ectoderm becomes reduced to a thin cord (the 

 ductus ecto-entobranchialis III), which eventually disappears. The dorsal 

 part of the pouch retains its lumen, its ventral part on the contrary becomes 

 solid and grows ventrally as a solid prolongation. Over the cranial wall of 

 its dorsal portion the cells assume a regular columnar arrangement and stain 

 deeply with eosin. This part constitutes the primordium of epithelial 

 body III. Over the remainder of the pouch, including the entire caudal 

 wall of its dorsal part and the solid ventral prolongation, the cells assume 

 a less regular, looser arrangement and stain rather less deeply. This portion 

 constitutes the primordium of thymus III. The two primordia so differentiated 

 soon separate from each other. Epithelial body III, at first luminated,. 

 becomes solid and moves slightly forwards to take up its permanent position 



