436 DR THOMAS H. BRYCE ON 
the elements is rather different, the essential problem is the same in both lower and 
higher animals. The heterogeneity of the erythrocyte series through the whole larval 
life, and the fact that all the elements except the mature erythrocyte multiply by 
mitosis and after their kind, seems a presumption in favour of continued new formation. 
If this were so the erythroblasts would necessarily bear a relation to some less 
specialised cells. Analogous facts in other forms have led to the theory, in all its 
varieties, of the origin of the red cells from the white elements. 
From the theory that the vascular and lymphatic systems are formed in the 
mesenchyme (ZIEGLER), whatever view may be taken of the actual channels and spaces, 
it follows that the cells, fixed and free, red and white, are all homologous, having the 
same parentage. Further, the theory gives room for the acceptance of continuous 
formation of free cellular elements, at any rate up to the completion of histological 
differentiation, and of the genetic relation in some sort between the red and white 
corpuscles. 
On the other hand, if the vessels arise 7 setu but the blood has a restricted local 
origin in one part of the embryo from which the corpuscles are dispersed—as pointed to 
by the works of Rtckerr, Rasy, Zwinck, SwaEn and BracuetT—and the sites of origin 
are not also those of the germs of the connective tissue, the blood and the lymph may, 
from the embryological and morphological point of view, require to be distinguished 
(Ferrx, Swaen and Bracuer). The two classes of corpuscles might then belong to 
two separate stirps, multiplying by division and having no mutual relationship. 
The origin of the first leucocytes has not yet been demonstrated beyond doubt, but 
the almost universal opinion has referred them to the mesoderm (mesenchyme). This 
is ZIEGLER'S* view, and in this country it has been specially maintained by GULLAND. 
They are ‘ wandering cells’ formed outside the blood stream; and as they appear at a 
later stage of ontogeny than the primitive blood corpuscles, they belong to a different 
category. 
In recent years, however, there has been a tendency to derive the lymphoid cells 
direct from the endoderm. KOLLIKER first described the epithelial cells of the thymus ~ 
as becoming converted into lymphoid cells; PRenant, ScuuLttzn, Maurer, NussBauM 
and Prymak, and Brarpy{ have come to a similar conclusion, and the last named 
claims for the gland that it is the sole source of the leucocytes. 
A similar conversion of the epithelial cells of the gut into the lymphoid cells of 
the intestinal glands has been described by Rerrerer, RupincER, Daviporr, and 
Kraatscn, but it has been denied by Stour and Koiimann.t 
In the matter of the spleen there has also been a question of the endoderm providing 
the cells of its rudiment. This view has been put forward specially by Maurer and — 
Kuprrer, but the older view that it is formed from the mesoderm as a mass of 
* Ber, der Naturforsch. Gesellsch. zw Freiburg, Bd. iv., 1889. Verhandl. d. deutschen Zool. Gesellsch., 1892. 
+ Zool. Jahrbucher Abt. f. Anat., vol. 17, 1903. For a historical account of histogenesis of thymus and references 
to literature this work may be consulted. 
t For critical review and references, see KonLMaNn, Archiv f. Anat., 1900. 
