THE THYMUS GLAND AND THE ADRENALS. 181 



in keeping with the collateral observations of a number of the 

 best of modern embryologists. 



The most prolific source of leucocytosis, as is well known, 

 is the bone-marrow. In the process of bone-formation during 

 foetal life, the first points of ossification appear during the 

 second month, but it is only during the fourth that the devel- 

 opment becomes markedly active on all sides. That the most 

 active work of the thymus is performed during intra-uterine 

 life is well known; this, therefore, coincides with its most 

 active bone-forming period. It seems reasonable to conclude 

 that so important a function as leucocytogenesis should not 

 devolve upon structures undergoing formation, and also that 

 the bone-forming organ should be intrusted with the function 

 which ultimately would constitute the main active attribute 

 of their product. Bone-marrow being the main leucocyte- 

 forming structure, we should therefore expect the thymus to 

 assume this role until the bone-marrow had reached its normal 

 physiological development. 



Kolliker has always maintained that the formation of 

 leucocytes was a function of the thymus: a position in which 

 he has been sustained by Prenaut and Oscar Schultze. J. 

 Beard 62 more recently took up the question and studied it with 

 considerable care in the Raid batis, the smooth skate. He 

 ascertained that the absence of leucocytes in the earliest period 

 of embryonic blood in vertebrates persists until the first ones 

 are formed within the thymus epithelium and from its epithe- 

 lial cells. In embryos from twenty-eight to forty-two milli- 

 meters long the formation and emigration of leucocytes from 

 the thymus becomes very active, and at this time there is no 

 part of the embryo, including the blood, that is not infiltrated 

 with leucocytes. This happens before lymphoid structures are 

 developed elsewhere within the body of the smooth skate. 

 Beard believes, with Kolliker, that the formation of leucocytes 

 is a function of the thymus gland; and "the first leucocytes 

 arise in the thymus from its epithelial cells; thus it is the 

 parent-source of all the leucocytes of the body": a conclusion 

 which is further sustained by what appears to us to be a war- 



62 J. Beard: Lancet, Jan. 21, 1899. 



