CELLULAR ELEMENTS OF THE MAMMALIAN EMBRYO 75 



sections stained with Gienisa or Azur-Eosin in accordance with 

 the technique developed by Maximo w. 



In the study of these cellular elements as they occur in the peri- 

 cardial, pleural and peritoneal cavities, the following precautions 

 are to be taken into account. In the case of early embryos in 

 which the peritoneal ca^dty is still in communication with the extra 

 embryonic coelom, if the umbilical cord has been cut previous 

 to fixation there is present the possibility of an accidental en- 

 trance of external elements into the peritoneal cavity through 

 the cut cord during the fixation and removal of the embryo from 

 the uterus During the necessary manipulation of staining 

 and mounting, extra coelomic blood and tissue cells may become 

 accidentally detached and transferred to the coelomic areas of 

 the section. In the present study in the case of a doubt as to 

 confusion with such dislocated cells the data was either discarded 

 or recorded with a question mark. In the majority of cases, 

 however, erroneous data arising from such sources can be satis- 

 factorily eliminated by confining the evidence to such cells or 

 groups of cells which can be demonstrated to extend through 

 two or more successive sections in the series, or are clearh^ em- 

 bedded in the coagulum of the serous fluid. The results of 

 such a critical study seem to leave no doubt as to the normal 

 and constant occurrence of a considerable number of free cel- 

 lular elements (somewhat variable perhaps in quantity) in the 

 body cavities of the mammalian embryo. 



These free cells are irregularly distributed throughout the 

 serous fluid. At certain stages of development, i.e., previous 

 to the closure of the pleiu-o-pericardial and pleuro-peritoneal 

 canals, they may be especially abundant in the region of the 

 developing pleuro-pericardial and pleuro-peritoneal membranes 

 In general they are not infrequently found relatively more 

 numerous and aggregated at one side or another of the cavity, 

 a condition no doubt due in part to the settling of the coelomic 

 fluids and their cellular content toward one side of the body 

 during fixation, as is not infrequently observed in the case of the 

 blood in the heart and blood vessels. 



