T 
io 6) 
The Development of the Thymus 
The author apparently believes that none of the small round cells leave 
the gland though he admits that lymphocytes enter. But as mentioned above 
the zone of connective tissue immediately around the head at 7 cm. may 
contain even more lymphocytes than are present inside the gland at that time 
If these are all entering the gland then it is probable that most of the small 
round cells are really lymphocytes. This conception then does not simplify 
the problem but is only a theoretical compromise between the two views as 
to the origin of the lymphocytes. 
J. Aug. Hammar (Zur Histogenese und Involution der Thymusdriise, Anat. 
Anz. Bd. XXVII, June 17, 1905) regards the reticulum as formed from the 
epithelial anlage but thinks the evidence at hand insufficient to decide the 
question as to the origin of the lymphocytes. He finds lymphocytes outside 
the thymus in many animals (man, cat, chick, frog) before any are present 
inside the gland. The corpuscles of Hassall develop from the epithelial 
reticulum and undergo hyaline (colloid?) degeneration. 
My description of the formation of the corpuscles of Hassall differs essen- . 
tially from Hammar’s, in that I believe the formation of the corpuscle consists 
in the expansion of the cytoplasm of the syncytium and its conversion into 
colloid. Hammar did not recognize ‘“‘ colloid in formation,’ though he speaks 
of the coarse fibrillar structure of the protoplasm. He -did not describe such 
corpuscles as are shown in Fig. 7, Plate II. 
The considerations presented above in favor of the epithelial origin of the 
lymphocytes seem to me much stronger than those given by Hammar. His 
statements as to the presence of lymphocytes around the thymus before they 
are present inside are to be taken with some reservation inasmuch as he 
mentions small round cells separate from the syncytium earlier, but regards 
them as degenerating epithelial cells (S. 65). His figure from thé human 
foetus (Fig. 18, S. 66) does not seem to be strong support for his statement. 
Certainly many lymphocytes are present in the pig thymus when the reticu- 
lum is broken up as much as shown in the figure referred to. It is also to 
be borne in mind that the different parts of the thymus undergo the lymphoid 
transformation at different times and that a single section may therefore be 
misleading. 
LITERATURE. 
la. AFANASSIEW, B.—Ueber die concentrischen Kérper der Thymus. Archiv 
ie, monlkor, ANH OS, 1x6, ROY. IST 
Weitere Untersuchungen tiber den Bau und die Entwickelung 
der Thymus und der Winterschlafdriise der Séiugethiere. Archiv f. 
amdcar, ANNES, JBL, DOIN 5 Weir 
AMMANN, A.—Beitrige zur Anatomie der Thymusdriise. Basel, 1882. 
2a. BEARD.—The development and probable function of the thymus. Anat. 
Anz., Bd. IX, 1894. 
The true function of the thymus. Lancet, 1899. 
4. Born, G.—Ueber die Derivate der embryonalen Schlundbogen und 
Schlundspalten bei Séugethieren. Archiv f. mikr. Anat., Bd. XXII, 
1883. 
5. Cornin et Ranvrer.—Manuel d’histologie pathologique. Paris, 1869, p. 
135 (cited from Ammann). 
1b. 
