DEVELOPMENT OF THE THYMUS 319 



of the pig arises by a constriction of the fundus praecervicaHs, 

 but claims that in embryos up to 35 mm. in length, the oldest 

 stage he examined, its histological structure does not resemble 

 that of the thymus more than any other branching epithelial 

 mass. 



Born ('83) derived the thymus anlage in pig embryos from the 

 third pharyngeal pouch, while Bell ('06) also is inclined to believe 

 that the ectoderm takes no part in the formation of the thymus. 



MATERIAL AND METHODS 



For investigation of the early stages of the morphogenesis of 

 the thymus, the excellent collection of pig embryo series, 3 to 

 42 mm. in length, in the Department of Histology and Embry- 

 ology of Cornell University, proved very helpful. In addition 

 to these, five embryos ranging from 9 to 21.5 mm. in length, 

 and the neck and upper thoracic region of eight embryos ranging 

 from 32 to 95 mm. in length, were sectioned transversely. These 

 sections, 10 microns in thickness, were stained with hematoxylin 

 and eosin. From series of this group reconstructions of the 

 pharyngeal region were made. Many dissections exposing the 

 thymus were made of the neck and upper thoracic region of 

 embryos from 100 to 280 mm. in length (full term). The thy- 

 mus of a pig one day old was also examined. 



MORPHOGENESIS 



In the investigation on the morphogenesis of the thymus special 

 attention was constantly directed toward the development of 

 the superficial thymus because its existence is not yet generally 

 accepted and, since the latest developmental stage in which it 

 was investigated by Zotterman was only 105 mm. in length, its 

 fate is not definitely known. 



An 11 mm. embryo was the developmental stage chosen as 

 the starting point for the study of the morphogenesis of the 

 thymus. At this stage the ectodermal and entodermal parts 

 of the branchial grooves and pharyngeal pouches can still be 



