282 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



side of the dorsal mesogastrium near the dorsal pancreas (Fig. 288 A). The 

 thickening is due to a temporary proHferation and invasion of mesothehal cells 

 into the underlying mesenchyme, which, meanwhile, has also undergone local 

 enlargement and vascularization. Contrary to the older view, the cells from the 

 peritoneal epithelium probably give rise to a large part, if not all, of the future 

 spleen. The splenic anlage becomes pinched off from the mesogastrium (Fig. 288 

 B) with which it is ultimately joined by a narrow band only. 



At first the blood vessels constitute a closed system. The peculiar adult 

 circulation is acquired relatively late. Lif schitz has shown that, in human fetuses 

 between 150 and 300 mm. long, red blood cells are actively formed in the splenic 

 pulp as clusters around the giant cells. The lymphoid tissue of the spleen first 

 appears as ellipsoids about the smallest arteries in fetuses of four months. At 

 seven months the ovoid splenic corpuscles appear as l3Tnphoid nodules about the 

 larger arteries. 



Glomus Coccygeum. — The coccygeal gland is present in 150 mm. (C H) 

 fetuses as an encapsulated cluster of polyhedral cells at the apex of the coccyx. 

 Later it becomes lobulated by the ingrowth of connective tissue trabecule and 

 receives a rich vascular supply. According to Stoerck (1906) its tissue at no time 

 resembles the chromaf&n bodies, although this has commonly been beheved. 



