611 



cardinal vein. Figure 1 is from a sagittal section of a rabbit em- 

 bryo of 13 days and shows the sinusoids with their endothelial lining. 

 In rabbits of 14 days the sinusoids have become smaller, probably 

 owing to the rapid growth of the surrounding parenchyma. An artery 

 can now be seen coming ofi from the aorta just above the suprarenal 

 and running to the outer side of it, where a branch joins directly 

 with the sinusoids. This is shown in Figure 2. The sinusoids still 

 communicate freely with the cardinal vein. The artery passes on and 

 is lost in the substance of the mesonephros. 



Fig. 1. Embryo rabbit of 13 days. 

 Suprarenal, part of sagittal section; 

 Si sinii^oid ; b blood corpuscle. Har- 

 vard Embryological Collection, no. 153. 



Fig. 2. Embryo rabbit of 14 days. 

 Supra-i'enal, part of sagittal section ; 

 a arteiy ; Si sinusoid. Harvard Em- 

 biyological Collection, no. 156. 



Fig. 1. 



Fig. 2. 



In older rabbit embryos the cells of the parenchyma become com- 

 pactly arranged, the sinusoids are diminished in size until they re- 

 semble true capillaries and their connections with the vein are re- 

 duced in number, until only one or two large openings are loft. (Har- 

 vard Coll., no. 170, sections 1571 et seq.) 



From this account it will be seen that the artery, at an early 

 stage, becomes connected directly with the sinusoids of the suprarenal, 

 which become rapidly reduced in size until they approach the size 

 and appearance of capillaries, at the same time losing most of their 

 connections with the vein. This method of establishment of the ar- 

 terio-venous circulation will probably be found to occur also in the 

 mesonephros. 



Harvard Medical School, June, 1901. 



39* 



i 



