316 



EMBRYOLOGY. 



Birds the two fundaments fuse early upon either side of the trachea 

 into a longish tract of tissue, which in the former is shorter 

 (fig. 177 A), but in the latter very much elongated (fig. 177 B). 



In Mammals it is principally the third visceral cleft which con- 

 tributes to the formation of the thymus. According to KOLLIKER, 

 BOBN, and RABL this is the only one which comes into considera- 

 tion, whereas DE MEURON, KASTSCHENKO, and 

 His give an account which differs from this, 

 but only in minor details. 



The further changes of the fundament of the 

 thymus in Mammals and in Man may be briefly 

 summarised as follows. The thymus-sac, which 

 probably takes its origin from the third visceral 

 pocket, encloses only a very narrow cavity, but 

 possesses a thick wall composed of many elon- 

 gated epithelial cells (fig. 178). It then grows 

 downward toward the pericardium, and at the 

 posterior end begins to form, like a botryoidal 

 gland, numerous rounded lateral branches (c). 

 (KOLLIKER.) These are from the beginning of 

 their formation solid, whereas the sac-like part 

 (a), which occupies the neck-region, always 

 continues to exhibit a narrow cavity. 



The budding continues for a long time, and 

 meanwhile extends to the opposite end of the 

 originally simple glandular sac, until the whole 

 organ has assumed the lobed structure peculiar 

 to it. At the same time an histological meta- 

 morphosis is also taking place. Lymphoid 

 connective tissue and blood-vessels grow into 

 the thick epithelial walls and gradually destroy 

 the appearance which so resembles a botryoidal 

 gland. With the increase in the size of the 

 organ the lymphoid elements coming from the 



surrounding tissue predominate more and more ; the epithelial rem- 

 nants are finally to be found only in the concentric bodies of HASSALL, 

 as MAURER has shown for Bony Fishes and as His has undoubtedly 

 rightly inferred for Man and Mammals. The cavity originally present 

 and resulting from the in vagina tion disappears, and instead of it 

 there arise new irregular cavities, probably the result of a breaking 

 down of the tissue. 



-Fig. 178. Thymus of an 

 embryo Rabbit of 16 

 days, after KULLIKEB. 

 Magnified. 



a, Canal of the thymus ; 

 b, upper, c, lower end 

 of the organ. 



