336 



TEXT-BOOK OF EMBRYOLOGY. 



FIG. 300. Hassall's corpuscle from 

 the thymus of a human fcetus of 

 70 mm. Hammar. 



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. The latest researches 

 still leave the origin of the lymphoid cells in 

 doubt. They tend, however, to show that 

 the epithelium is transformed into the 

 reticular tissue of the thymus, in which the 

 lymphoid cells, whatever their origin may 

 be, undergo mitotic division. Hassall's 

 corpuscles possibly represent compressed 

 parts of the reticulum (Fig. 300). 



THE .GLOMUS CAROTICUM. The early 

 formation of the glomus caroticum (carotid 

 gland) has not been observed in the human 

 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. 318), 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. 362). The latter is lined with, 

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

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

 thorax (p. 247), 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 be 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 anal, 

 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. , 



