CELLULAR ELEMENTS OF THE MAMMALIAN EMBRYO 81 



the macrophags may undergo mitosis even while still retaining 

 incompletely transformed remnants of previously ingested mate- 

 rial in its cytoplasm (fig. 11). 



2. Evidence as to their origin from the coelomic mesothelium 



a. -Certain characteristics of the mesothelium in general. Are 

 these macrophags of the coelomic cavities cells which have mi- 

 grated into these body spaces from the neighboring blood vessels 

 or are they tissue cells which have become detached or liberated 

 from the tissue walls surrounding these cavities where they have 

 undergone further differentiation and assumed phagocytic and 

 possibly other functional activities in the serous fluids? As to 

 the first view, while no conclusive evidence of such a migration 

 of macrophags from the blood vessels and adjacent tissues was 

 obtained, it is to be recognized as not improbable that such cells 

 may enter the embryonic coelom in this manner as has been 

 maintained by Maximow to occur in the serous cavities of even 

 the adult mammal. But the crucial question still remains as 

 to whether this is to be regarded as the only source of the coe- 

 lomic macrophags in the embryonic body cavities. 



The embryonic mesothelium consists of cells which are rather 

 flattened in form. The nuclei also have a correlated flattened 

 oval shape and cell walls are not clearly evident. Tj^ically 

 these cells form a single epithelial layer lining the coelomic cavi- 

 ties {mes in figs. 15 to 18 and 41 to 44). Such a layer is, however, 

 by no means always sharply defined, for in various regions the 

 surface cells are in such an intimate syncytial association with 

 the deeper lying cells that characteristic structural differences 

 between them are not readily evident, indeed in certain regions 

 the conditions are such as to suggest that the surface meso- 

 thelial cells may have given rise to many deeper lying cells 

 comparable to the endothelial growths described by Mall ('12, 

 pp. 258, 261) in certain endocardial regions of the heart. In a 

 careful study of the mesothelium as seen in serial sections, it 

 may be noted that the form and structure of its component 

 cells are not always constant throughout the body cavities. In 

 various regions the flattened mesothelial cell body, as well as 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 20, NO. 1 



