GEKMINATINO ENDOTHELIUM. 7 



complete it to the extent that there exist groups or tracts of germi- 

 nating endothelium in rabbits as well as in guinea-pigs, cats, dogs, 

 and monkeys, which, as we shall see afterwards, stand in an intimate 

 relation to lymphatic vessels. 



Somewhat different in its arrangement is the germinating endo- 

 thelium of the mesentery of frogs, cats, dogs, and monkeys, viz. in 

 these membranes they occur only in groups of less than five endo- 

 thelial cells, mostly two or three. They are also of a polyhedral 

 shape, consisting of a distinctly granular protoplasm, with a con- 

 stricted or double vesicular nucleus ; their size is considerably smaller 

 than that of the common endothelial plates surrounding them. On 

 fresh preparations, which have been mounted in serum with great 

 care, they appear as granular bodies projecting from the surface like 

 buds ; on preparations stained with silver solution, they appear 

 generally beset with brownish granules. There exists a great variety 

 as regards the number of such groups in a given field. Searching the 

 surface of the membrane under a magnifying power of about 200, 

 one passes two or three fields where there are no groups ; whereas, in 

 a neighbouring field one meets with half-a-dozen or even a dozen of 

 them. They are to be found over those parts which contain large 

 blood-vessels quite as often as over the intermediate portions. 



These structures are not to be confounded with those very small 

 figures which occur on silver-stained preparations, isolated or in 

 small groups, amongst the large endothelial fields : a part of those 

 small fields may correspond to young endothelial cells if they are 

 distinctly granular. 



If one examines carefully the surface of a silver-stained mesentery 

 of the frog, one will find that the large common endothelial plates, 

 which in intensely-stained preparations show a clear nucleus in a 

 brownish cell substance, do not touch each other everywhere by 

 means of the well-known more or less wavy lines, but that these 

 silver lines in some places appear to be replaced by a branched cor- 

 puscle, the body of which is situated at the point of junction of a 

 number of endothelial cells, and the processes of which stretch 

 between these endothelial cells so as to become identified with their 

 silver lines. 



