DEVELOPMENT OF THE THYMUS 325 



The anterior portion of the thymus (T.e.) has lost its connec- 

 tion with the pharynx and hes on the dorso-lateral side of the 

 parathyroid and is fused with the vesicula cervicalis medialis. 

 In the region of the fused portion it contains a cavity of con- 

 siderable size while the remaining portion along the epithelial 

 body is without a lumen. From the epithelial body the 

 thymus extends in a caudal and a slightly medial and ven- 

 tral direction as a solid cord of cells. Just anterior to its 

 entrance into the thoracic cavity it is slightly enlarged. The 

 extreme caudal portion which lies within the thoracic cavity 

 turns abruptly in a ventral direction, is greatly flattened, and 

 in contact with the pericardium. The thoracic segments of the 

 right and left thymus at this stage lie closely together but are 

 not fused. Bell, however, in a 20 mm. embryo, describes them 

 as being fused. 



The hypoglossal nerve now forms an acute angle with that 

 portion of the vagus lying immediately posterior to it. In the 

 two preceding stages that were modelled, the corresponding 

 angle formed by these two nerves was obtuse instead of acute. 

 This change in the form of angle between the earlier and later 

 stages apparently is due to shiftings — a consideration of which 

 is to follow — that take place in the neck during the growth of 

 young embryos by which a stress appears to be exerted on the 

 hypoglossal nerve by the cervical vesicle. 



The thymus at this early stage (21.5 mm.) can be divided into 

 seven regions, most of which in the later stages become very 

 pronounced. They are: (1) The 'superficial thymus' which rep- 

 resents the vesicula cervicalis lateralis and is of a purely ecto- 

 dermal origin; (2) the 'thymus head' which represents the struc- 

 ture formed by the fusion of the vesicula cervicalis medialis and 

 the anterior portion of the entodermal anlage of the thymus; 

 (3) the 'connecting band' which loops over the hypoglossal nerve 

 and connects the superficial thjanus with the thymus head and 

 is of a purely ectodermal origin; (4) the 'mid-cervical segment' 

 which is an enlargement of the thymus between the intermediary 

 and cervico-thoracic cords; (5) the 'intermediary cord' which 

 connects the thymus head with the mid-cervical segment; (6) 



