74 V. E. EMMEL 



I. INTRODUCTION 



The following study is concernad with the free cellular ele- 

 ments in the pericardial, pleural and peritoneal body cavities. 

 The results of recent morphological as well as experimental and 

 clinical investigations regarding the nature and origin of these 

 structures as they occur in the serous cav'ties of the adult mam- 

 mal are of a divergent character. In view of this fact together 

 with the apparently entire absence as yet of embryological data 

 bearing on the problem, it has appeared desirable to obtain if 

 possible more definite information as to the cytological condi- 

 tions in the coelomic cavities of the embryo. The nature of the 

 present subject has also necessitated the extension of the study 

 to include certain cellular structures occurring in the embryonic 

 mesenchyma. 



Part of the following study was made while at the University 

 of Strassburg during a leave of absence from the Washington 

 University Medical School. I wish to express my indebt- 

 edness to Prof. G. Schwalbe for the generosity with which the 

 facilities of the Anatomical Institute were placed at my disposal 

 and the encouraging interest taken in the work by Prof. Franz 

 Weidenreich. It is with regret that in consequence of the pres- 

 ent disrupted political condition in Europe it has been neces- 

 sary to forego the pleasure of Professor Weidenreich's valued 

 criticism of the final results of the research, 



II. OCCURRENCE AND DISTRIBUTION OF THE FREE CELLULAR 

 ELEMENTS IN THE EMBRYONIC COELOM 



The presence of free cellular elements in the embryonic body 

 cavities were first noted in a 7.4 mm. pig embryo. In order to 

 ascertain to what extent these elements were normally present, 

 the observations were subsequently extended to include the fol- 

 lowing material: 5 to 12 mm. pig embryos, 9 mm. rabbit em- 

 bryos, 5 to 9 mm. mouse embryos and several 9 mm. opposum 

 embryos. The specimens were fixed in Zenker-formalin (Helly's 

 modification), embedded in celloidin or paraffin and the serial 



