DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 335 



mation," asserting that the epithelial cells were transformed into lymphoid 

 cells in situ, and that Hassall's corpuscles were remnants of epithelium and 

 disintegrating blood vessels. Some went even so far as to assert that 



the thymus was the first place of origin of 

 the leucocytes. More recent researches 

 furnish very strong evidence that no lymph- 

 oid cells are derived from the epithelial 

 cells (Maximow), but that the epithelium is 

 transformed into the reticular tissue of the 

 thymus, in which the lymphoid cells undergo 

 mitotic division, Hassall's corpuscles possibly 

 representing compressed parts of the reticu- 

 lum (Hammar) (Fig. 300). 



THE GLOMUS CAROTICUM. The early 

 formation of the glomus caroticum (carotid 

 FIG. 300. Hassall's corpuscle from gland) has not been observed in the human 

 ^mnr^lr. 1 " 11311 ^'" 50 ' embryo. From observations on lower 



animals it has not been made clear whether 



it is derived from the entoderm of a branchial groove or from the adventitia 

 of the carotid artery. 



The (Esophagus and Stomach. 



THE (ESOPHAGUS. When the primitive gut becomes differentiated into 

 distinct regions (p. 317), the cesophageal region forms a comparatively short 

 tube, of uniform diameter, extending from the pharynx to the stomach (Fig. 

 285). In embryos of about 3 to 4 mm. the anlage of the respiratory system 

 arises from the cephalic end of the tube (see p. 360). The latter is lined with 

 entoderm and broadly attached to the dorsal body wall by mesoderm (Fig. 285). 

 During later stages it becomes relatively longer as the heart recedes into the 

 thorax (p. 245), but maintains its uniform diameter. 



Further development produces no marked changes in the relative position 

 of the oesophagus. It remains broadly attached to the dorsal body wall 

 throughout the life of the individual. In other words, there is never a distinct 

 mesentery. The entoderm gives rise to the epithelial lining and the glands, the 

 surrounding mesoderm to the connective tissue and muscular coats. 



THE STOMACH. The anlage of the stomach can br recognized in embryos 

 of about 5 mm. as a slight spindle-shaped enlargement of the primitive gut a 

 short distance cranial to the yolk stalk (Fig. 284). The dilatation goes on more 

 rapidly on the dorsal than on the ventral side, thus producing the greater and 

 lesser curvature respectively. The greater curvature is attached to the dorsal 

 body wall by the dorsal mesogastrium which is a part of the common mesentery. 



