CELLULAR ELEMENTS OF THE MAMMALIAN EMBRYO 103 



VI. CONCERNING THE PRESENT STATUS OF THE QUESTION AS TO 



THE ORIGIN OF MACROPHAGS FROM THE COELOMIC 



MESOTHELIUM 



1. Emhryological and comparative 



As already intimated, a study of the cellular elements in the 

 embryonic coelom of mammals has not been previously made. 

 At the same time it may be observed that the present conclu- 

 sion that the coelomic epithelium may give rise to free functional 

 elements in the coelomic cavities is indirectly supported in an 

 interesting manner by the results of a number of recent inves- 

 tigations. Reference may be made to Bremer's ('14) work in 

 which he finds anlages of the earhest blood vessels in man to 

 arise from the surface mesothelium. His "observations point 

 to the ingrowths of the mesothelial layer covering the yolk-sac 

 and body-sac as the anlages of the blood vessel endothelium 

 and of a lesser extent of the blood corpuscles" (p. 459), and the 

 conclusion is drawn that "True blood islands may occasionally 

 arise by the multiplication of the cells of the mesothelial in- 

 growths, or scattered blood corpuscles may arise singly' within 

 these ingrowths" (p. 464). For the cat embryo Schulte ('14) 

 records the occurrence of "funnel-like diverticuli of the coelom, 

 the walls of which are intimately united to the blood vessels," 

 and which he suggests may be of morphological significance 

 with reference to the development of the embryonic blood ves- 

 sels (p. 80). Haff ('14, pp. 346 and 333) states the conclusion 

 that the peritoneum covering the embryonic liver of the chick 

 may give rise to cells within the liver differentiating into eryth- 

 rocytes. Scammon ('15), in the histogenesis of the Selachian 

 liver, records the occurrence of mesothehal tubules "the walls 

 of which are continuous with the splanchnic mesothelium and 

 the lumen with the coelomic cavity" (p. 276). Although un- 

 able to find that the lumen of these tubules connected with that 

 of the blood spaces, it is stated that the tubules break up into 

 mesenchymal strands and that "the mesenchymal and endothe- 

 Hal cells form free anastomosis" (p. 280). Phylogenetically ref- 

 erence may also be made to primitive vascular conditions in 



