CELLULAR ELEMENTS OF THE MAMMALIAN EMBRYO 83 



just described are at least not to be discarded as being merely 

 misleading appearances due to tangential planes of section as 

 can be ascertained from the serial sections. The possibility 

 was also considered as to whether such cell forms may not be 

 due to a shrinkage or folding of the coelomic walls or artificial 

 ruptures of the mesothelial surface. All of these artificial con- 

 ditions may of course occasionally occur in consequence of the 

 histological technique employed but the rounded projecting 

 cells do not have the appearance of elements artificially torn 

 from the living mesothelium. If they are the products of his- 

 tological shrinkage, it is not easy to understand how single iso- 

 lated cells could be made to assume the present forms, for such 

 cells may be found in regions of the coelomic wall where the reg- 

 ular surface curvature is not indicative of any artificial irregu- 

 larity in its reaction to fixing agents. When found in regions 

 where the surface of the wall appears more irregular, the cell 

 types in question may occur on both convex areas of the slight 

 surface elevations as well as in the concave areas of the meso- 

 thelial depressions. What appears a criticism of a more seri- 

 ous character, however, is the possibihty that the cell forms 

 under consideration are either instances of cells from the adja- 

 cent mesenchyma and blood vessels migrating through the meso- 

 thelium to reach the coelomic cavities or else coelomic macro- 

 phags themselves merely resting upon or in close proximity to 

 the mesothehal surface. That a coelomic macrophag may occa- 

 sionally be caught in the coagulum of the serous fluid and fixed in 

 close contact with the coelomic surface must be granted. Fur- 

 thermore since one is apparently forced to admit that erythro- 

 cytic elements, as will be subsequently described, must enter 

 the coelom from extra coelomic regions it seems necessary to 

 admit the possible migration of other cellular elements into the 

 embryonic coelom. Much of the evidence also tends to be viti- 

 ated by the absence of decisive cytological differences between 

 many of the mesothelial cells and either the coelomic macro- 

 phags or similar cells occurring in the vascular channels. 



In view of the above difficulties it becomes necessary to ascer- 

 tain whether there exists anv other sources of evidence condu- 



