464 JOHN BEARD, 



In 1891 appeared two publications by Gulland. In some respects 

 his work marks a considerable advance, and in two directions. As 

 previously indicated, Gulland advanced and defended the thesis of 

 the unity of the leucocytes, and in so doing he raised them to the 

 rank of a distinct morphological category of cells. In this way he 

 paved the way for the recognition and later discovery, that there is 

 one organ, and one only, whose function is the production of leuco- 

 cytes. In his researches upon the development of the thymus, chiefly 

 of the rabbit, he, like Kölliker, found, that there is a period of the 

 development, during which the blood contained only coloured corpuscles, 

 and he stated, that the first leucocytes appeared in the connective 

 tissue around the thymus (p. 173). From this point his researches 

 must be described as erroneous, for like certain earlier observers he 

 believed, that these first leucocytes, derived by what Gulland terms 

 a condensation of connective tissue, then invaded the thymus. His 

 fig. 10, showing what he holds to be this invasion, resembles some- 

 what certain of the figures of the present work, where nests of leuco- 

 cytes are depicted within the epithelium. He derives the concentric 

 corpuscles from the original epithelial cells, but no attempt is made 

 to prove this. Nor would Gulland appear to have noted, that in 

 the latest periods of uterine gestation the thymus of the rabbit is to 

 all appearance made up almost entirely of leucocytes, and that even 

 then no concentric corpuscles are present. 



From the point of view of the present writing the great fact due 

 to Gulland's work is the recognition, that the first leucocytes appear 

 in the immediate neighbourhood of the thymus. 



Gulland's researches form the termination of the series, in which 

 the conversion of the thymus into a lymphoid organ was believed to 

 be brought to pass by an invasion of leucocytes into it. Without a 

 single exception the investigations since 1892 have led to the result, 

 that the original epithelial cells become converted into leucocytes. 



Prenant's memoir, which appeared in 1894, is a very important 

 one in this respect, and it is the first of a series, in which the stages 

 of the histogenesis are followed out. His work treats of the develop- 

 ment and histogenesis of the thymus of the sheep. Its origin as an 

 epithelial tube is described, and this condition is found to persist, 

 until the foetus is 28 mm in length. Prenant assigns the trans- 

 formation of the epithelial thymus into the lymphoid one to the 

 period of 25 to 85 mm "embryos". As the embryonic development 

 is finished prior to attainment of the size of 25 mm in the sheep, 



