460 JOHN LEWIS BREMER 



the endothelium and the mesenchyina through which it runs points 

 to a difference in origin between the two tissues. 



We have not yet attained a sure histological basis for differen- 

 tiating endothelium in ordinary specimens, nor is it perhaps to be 

 expected in tissues as young as those under discussion. In a 

 recent paper Clark^^ described certain differential characteristics 

 of endothelial nuclei, after special fixation and stains, but this is in 

 chicks of a relatively older stage than the present human material, 

 and I could find no trace of such differences in younger material 

 and with the common stains used. In the Grosser embryo, as 

 shown in many of the drawings, endothelium and mesothelium are 

 both often marked by the presence of very fine intra-cellular fibrils, 

 absent in the mesenchymal cells ; these fibrils are not found in the 

 Minot embryo, nor in any of the others studied, owing probably to 

 differences in the fixing fluid employed. They cannot, therefore, 

 be used as a general distinguishing sign. 



The coincidence of the views forced upon me by my observations 

 with the now ancient theory of His,^^ Butschli,^"* and others, that 

 blood-vessels are in some way related to the coelom, is apparent, 

 and the significance of this when correlated with the facts known 

 of the inter-relation of the blood-vascular system and the coelom in 

 certain invertebrates, must strike anyone who is interested in 

 phylogeny. 



This has been briefly noted by Hungtington^^ who found, in a 

 cat embryo of 10 mm., a ''clearly limited and well defined funnel- 

 shaped stoma, occupying the dorsal extremity of the coelomic cleft 

 and apparently opening directly into the spaces of the early lym- 

 phatic plexus" (p. 26), without, however, attaching more than a 

 "suggestive" importance to the fact. It is also interesting to note 

 that the origin of endothelium from the mesothelium adds to the 

 already great number of structures derived by ingrowths from this 

 source, among which may be mentioned the tubules of the pro- 



12 Clark, E. R. Anat. Record. V.ol. 8, p. 81, 1914. 



1' His, VV. Abhandl. math. phys. Classe K. Sachs. Ges. Wiss. Vol. 26, p. 173, 

 1900. 



1^ Blitschli, O. Morph. Jahrb. Bd. 8, p. 474, 1883. 



15 Huntington, G. S. Memoirs Wistar Institute, No. 1, 1911. 



