298 RICHARD E. SCAMMON 



7. The primitive embryonic type of nuclear structure is lost 

 in the mesenchymal cells as they are proliferated from the meso- 

 thelium. No sharp line of demarcation could be drawn between 

 endothelial and mesenchymal cells. 



D. Hepatic sinusoids 



1. Hepatic sinusoids arise in selachians by intercresence of 

 the hepatic cylinders with the omphalo-mesenteric vein. This 

 intercresence may be brought about either (a) by the growth of 

 the rami of the omphalo-mesenteric vein about the cylinders as in 

 Acanthias, or (b) by the invagination of the vessel wall by grow- 

 ing cylinders as in Mustelus and Torpedo. 



2. In the first type there may be recognized three stages of 

 sinusoid development: (a) The first in which the sinusoids are 

 sparse and differ from capillaries only in their terminations at 

 either side in veins; (b) The second stage in which the sinusoids 

 are greatly enlarged constituting approximately 50 per cent of 

 the liver; (c) The final stage in which the sinusoids take the form 

 of the capillary sinusoids of Minot. In the second type of inter- 

 cresence only the last two stages are present. 



3. The reduction of the sinusoids from stage (b) to stage (c) 

 is due to the growth in size of the hepatic cylinders and not due 

 to their increase in number, for the most active increase in num- 

 ber of cylinders takes place prior to or early in stage (b). 



