20 



conveying a clear plasmatic fluid without cellular contents. 

 The same picture is presented in the earliest stages of haemal 

 development in vertebrates. Thoma's 28 investigations of the 

 histogenesis of the blood vascular system in chick embryos have 

 furnished us with a very clear picture of the process. 



The first histogenetic inception of the vertebrate haemal 

 vascular system is marked by the condensation of the mesoderm 

 into cellular strands. Between the cells of these praevascular 

 strands multiple oval or round spaces develop, which enlarge, 

 elongate and become confluent, forming a network of inter-com- 

 municating channels, the hcemal capillary anlages. These chan- 

 nels contain a clear colourless fluid, with no, or only very scat- 

 tered, cellular elements. This fluid, obtained by secretion from 

 the free surfaces of the cells limiting the spaces, is evidently 

 under a certain definite pressure, which exerts an influence on 

 the form of the cells lining the channels. These limiting cells 

 lose their earlier isodiametric, more or less regular cuboidal form, 

 and appear flattened, and on optical section spindle-shaped. 

 They have begun to assume the endothelial character. Hence 

 from its earliest inception the endothelial lining of vascular 

 channels appears as an environmental adaptation of the meso- 

 dermal cell. One surface of a cuboidal cell is freed from con- 

 tact with adjacent cells by the development of an intercellular 

 cleft, and this free surface is subjected to the pressure of the 

 fluid contained in the earliest capillary anlages, modified by the 

 tension pressure of the organism as a whole. This mechanical 

 adaptation to the altered cellular milieu results in the formation 

 of endothelium, and the process is identical in all portions of 

 the mesoderm, independent of the question as to whether the 

 resulting endothelial lined space shall subsequently be incor- 

 porated in a haemal or a lymphatic system of vascular channels, 

 or shall remain as a closed non-vascular mesodermal space. 



It seems to me futile to try to speculate on an ontogenetic 

 stage in which endothelium acquires a "specific" character. It 



28 R. Thoma: " Untersuchungen tiber die Histogenese und Histomechanik des 

 Blutgefasssystems." Stuttgart, 1893. 



