THE DEVELOPMENT OF THE VASCULAR SYSTEM. 285 



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. 



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 chromaffin 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 con- 

 sidered 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 neces- 

 sarily imply the absence of the heart in the affected twin, for the latter may 

 develop to a considerable degree and possess a functionating heart. On the 

 other hand, the affected twin may be only an amorphous mass of tissue which 

 derives its total blood supply through the agency of the stronger twin's heart. 

 Or there may be any intermediate form between these two extremes. The 

 point is that the acardiac monster (acardiacus) derives its blood wholly or in 

 part through the agency of the stronger heart. A further discussion of acardiac 

 monsters and their possible explanation will be found in Chap. XIX. 



Double Heart. — But one or two cases of a double heart in a single human 

 foetus have been recorded. In some of the lower forms (chick) it occurs more 

 frequently. The explanation is probably to be found in the double origin of 

 the heart in Amniotes (p. 222). 



Anomalous Position of the Heart. — Congenital anomalies in the posi- 

 tion of the heart are rare. Dextrocardia (heart on the right side) is almost 

 invariably associated with changes in the position of the viscera (see transposi- 

 tion of the viscera, page 355). In the condition known as ectopia cordis, the 

 heart, with the pericardium, protrudes through a cleft in the ventral wall of 

 the thorax, the cleft being probably due to an imperfect fusion of the two sides 

 of the body wall in that particular region. 



Anomalies of the Septa. — The most frequent anomaly in the atrial 



