DEVELOPMENT OF THE MAMMALIAN SPLEEN 293 



By making injections into the dorsal aorta, Sabin revealed the 

 presence of both arterial and venous capillaries in the spleen of 

 a 3-cni. pig. After various but partially successful attempts at 

 injection into the umbilical vein of smaller embryos, the writers 

 resorted to a comprehensive study of serial sections and found the 

 latter far more satisfactory. While the injection method is 

 worthy of support, when successfully carried out, still the dif- 

 ficulty in its manipulation more than outweighs its merits, es- 

 pecially when applied to embryos from 7 to 10 mm. in length. 

 In embryos of that size, due to the non-resistant character of the 

 tissue, it is almost impossible to dissect out the aorta in order to 

 make a direct injection into the splenic artery, and if the can- 

 nula is inserted into the umbilical vein, the injection mass fails to 

 get beyond the larger branches of the aorta. 



Sabin reports the presence of both arterial and venous capil- 

 laries in the spleen of a 3-cm. pig, but makes no comments con- 

 cerning the circulation prior to that time. A study of serial 

 sections shows that the network is present in the early mesen- 

 chyme of the mesentery even before any marked differentiation 

 in the splenic rudiment has taken place. 



WTien the embryo reaches 4 to 6 cm. in length, a new relation 

 is established between the capillary vessels and the splenic mesen- 

 chyme. This change is preceded by the appearance of clear and 

 somewhat open areas in the mesenchymal tissue, in which dis- 

 tinct slits later become evident (fig. 3) . At the margins of the 

 slits, the mesenchyme cells still retain their irregular processes, 

 some of which extend well into the lumen of the sinus-like open- 

 ings. 



Many of the cells bordering these sinuses have nearly severed 

 their connection with the surrounding mesenchyme. The cyto- 

 plasm of some of them has become rounded and very basophilic, 

 while the chromatin of the nucleus has become rearranged, and 

 one or two prominent nucleoli have appeared. One such cell 

 is shown in figure 4. It lies within the sinus, but it is still con- 

 nected with the mesenchyme at the margin of the sinus by a 

 broad cytoplasmic strand. Excepting for its connection with 

 the mesenchyme, this cell shows all of the characteristics of the 



