256 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VoL. 11. 



by the more abundant supply of dissolved chromatin in the serum bath- 

 ing them. 



These hsematoblasts are met with most frequently in those parts of 

 the circulatory apparatus where the blood current is slow or where physical 

 conditions retard their movement. Such conditions are found between 

 the muscle trabeculae stretching through the heart cavity after these are 

 formed, in the concave portions of the aortic arches and especially in a 

 minute branch of the arteria mesenterica distributed in a plate of tissue 

 derived from the visceral layer of the mesoblast. This is the site for the 

 future spleen. The origin of the spleen in the visceral layer of the 

 mesoblast in the toad was pointed out by Goette* who described the 

 cells of the organ as direct descendants of the yolk cells (entoblastic 

 cells). My observations are not yet concluded in the development of the 

 spleen, but they have progressed so far as to allow me to say definitely 

 that the organ increases in bulk by multiplication of the capillaries 

 arising from the branch of th_' mesenteric artery to accommodate the 

 excessively large number of h?ematoblasts derived by division from the 

 original haematoblasts which have been caught in the narrow spaces of 

 the capillaries, early in development of the organ. At a date roughly 

 corresponding to the interval between the fortieth and sixtieth days, sec- 

 tions of the organ fixed in Flemming's Fluid and stained with haematox- 

 ylin and eosin, contain a very great number of elements like those repre- 

 sented in Figs. lo and ii. In fact sections of the organ thus prepared 

 have a deep ochre-red or terra-cotta-red color, owing to the great number 

 of mitotic haematoblasts present in it. At later stages of development 

 haematoblasts are rarel)^ found elsewhere than in the spleen, which is, from 

 now on, the organ for their production out of the original elements whose 

 history has been traced above and whose presence in the spleen is to 

 be explained as I have pointed out. Whether there is a secondary 

 formation of haematoblasts out of the cells of the original tissue of the 

 visceral layer of the mesoblast, it is impossible to say, but as the 

 haematoblasts and the spleen are both formed out of portions of visceral 

 layer, such a secondary origin is not, theoretically, improbable. All that 

 I can at present say is that early in the development of the spleen its 

 vascular channels become distended with haematoblasts, which are also to 

 be found in other vessels of the body where the blood current is slowed 

 or retarded, that these haematoblasts undergo rapid divisions and in- 

 crease thereby the size of the organ and that these divisions are quite 

 sufficient to explain the occurrence there of all the haematoblasts 

 observed. The first appearance of the organ in fact consists in the 



* Loc. cit. p. 812. 



