104 X V. E. EMMEL 



some of the lower invertebrates. Lang ('04, p. 152) states that 

 in the body cavities of anneHds there occur not only sex cells 

 but also amoebocytes (lymphocytes) and coelomocytes, some of 

 which as the result of formation of hemoglobin are designated 

 haemocytes, and that these elements arise from the coelomic 

 epithelium. Abbot ('13, p. 6) describes the observation of 

 hemoglobin containing cells or 'haematids' in the body cavity 

 of the Echiurid worm, Thallasema melhta, arising from the 

 "living membranes of the general body cavity which buds off 

 masses of cells, usually eight to twenty-four in number, which 

 ultimately break up into individual haematids" (p. 6). Data 

 of a similar character could be greatly extended not only among 

 annelids, but also in the Echinoderms and Coelenterates. The 

 views of Biitschh ('83), His ('00) and Arnold ('04) concerning 

 the phylogenetic origin of the circulatory system from the body 

 cavities are well known. 



2. In adult mammals 



In connection with these embryological results the question 

 arises as to whether these mesotheUal activities are confined to 

 the embryo, or whether such a potentiality may be retained 

 even in the adult animal. Without entering into a detailed ac- 

 count of the extensive hematological literature bearing upon the 

 much debated question as to the nature and origin of the free 

 cells in the adult serous cavities (for a discussion of which cf. 

 Weidenreich ('11, pp. 126-138), certain aspects of the problem 

 may briefly be considered in the light of the more recent inves- 

 tigations. 



Schott ('09), from cytological and experimental studies reaches 

 the important conclusion that the surface lining cells of the adult 

 body cavities in the guinea pig and rabbit are not highly special- 

 ized and fixed passive structures, without potentiality for fur- 

 ther differentiation, but that on the contrary they may assume 

 phagocytic activities, become detached from the serous mem- 

 brane and be liberated as free, active, living cells identical with 

 the lymphocytes and macrophags of these cavities. He states: 



