448 J. A. BADERTSCHER 



in the syncytium are somewhat more numerous than in the pre- 

 ceding stage. They vary greatly in size. Some are in contact 

 with the epithelial nuclei while others have no connection with 

 them. No consideration was given to the mode of their forma- 

 tion. They will again be considered in a later developmental 

 stage. 



Large lymphocytes, as in the previous stage, are found plenti- 

 fully in the mesenchj^me surrounding the thoracic segment of 

 the thymus. They are more numerous in the region of the larger 

 blood vessels of the thorax than in those parts of the connective 

 tissue containing only smaller vessels where, however, they can 

 be found without much searching. Around the superficial and 

 head thymus local accmnulations now occur. In general, they 

 are more numerous than in the joreceding stage. 



2. The epoch of lymphocyte infiltration and lymphocyte proliferation 

 and the formation of the reticulum 



SO mm. embryo. The thymus of this stage is decidedly in 

 advance of the 26 mm. stage just described. The lobules of 

 the superficial and head thymus and the cervical and thoracic 

 segments have greatly enlarged while those of the intermediary 

 and cervico-thoracic cords have started to develop. The mesen- 

 chyme occupies all the spaces between the lobules and is some- 

 what denser than that surrounding the thymus. Blood vessels 

 are numerous in the connective tissue septa but none are present 

 in the lobules. At this stage the lumen of the blood vessels is 

 comparatively large, and their walls are thin, being made up of 

 large endothelial cells only. 



The structure of the epithelial nuclei of the thymus is the 

 same as in the preceding stage. Mitoses (fig. 1, M.e.N., also 

 fig. 2, 37 mm.) are quite frequent. The "large dark nuclei" 

 and "small dark nuclei (lymphoblasts)," (figs. 1 and 2, D.e.N.), 

 according to Bell's nomenclature, are present as in the 23 mm. 

 embryo. These I regard as epithelial nuclei in the first stages 

 of degeneration. A completely degenerated epithelial nucleus 

 is also present (fig. l,D.e.X'). Its chromatin has massed into 



