THE DEVELOPMENT OT THE VASCULAR SYSTEM 285 



" spleen cells." The lymphocytes proliferate to a certain extent in the splenic 

 corpuscles, and in that way, at least, the spleen serves as a base of supply for 

 leucocytes. There is a possible suggestion that the first leucocytes of the 

 spleen have their origin in the mesenchymal cells of the spleen anlage. This 

 would be in accord with the observations which indicate that leucocytes are 

 derived from indifferent mesenchyme cells. 



Mesothelium Anlage of spleen 

 \ / 



Mesenchyme 



-- 



FiG. 260. From section through dorsal mesogastrium (anlage of spleen) of a chick embryo 

 of 3 days and 21 hours incubation. Tonkoff. 



Glomus Coccygeum. 



The coccygeal skein (coccygeal gland) was originally considered as belong- 

 ing to the same category as the suprarenal glands, but the latest researches 

 have indicated that its cells do not possess the characteristic chromafrin 

 reaction and that it belongs rather to the category of lymph glands. It 

 develops ventral to the apex of the coccyx in relation with branches of the 

 middle sacral artery. 



Although the thymus gland becomes a lymphatic structure it is primarily 

 derived from the epithelium (entoderm) of the branchial grooves and will be 

 considered in connection with the development of the alimentary tract (Chap. 

 XII). The tonsils also will be considered in the same connection. 



Anomalies. 



ANOMALIES OF THE HEART. 



ACARDIA. The malformation known as acardia occurs in the case of twins 

 that have but one chorion. The so-called acardiac condition does not 



