THE LYMPHATIC VESSELS OF THE SEROUS MEMBRANES. 55 



examine the development of lymphatic capillaries in the omentum of 

 the rabbit, and in the mesogastrium of the frog, as it may be easily 

 observed in those places. We have already become acquainted with 

 the fact that accumulations of branched lymph-canalicular cells form 

 the foundation of those peri-lymphangial patches and nodules which 

 are supplied with blood-vessels, as also of those which as yet are with- 

 out a blood supply. If we examine a number of such patches in suc- 

 cessful pencilled and silvered preparations, in which the number of 

 migratory cells is still inconsiderable, we may find, both amongst the 

 buds which spring from the lymph-canalicular cells, but still remain 

 connected with them, as also amongst the lymph-canalicular cells 

 themselves, some individuals in whose interior a vacuole is contained. 

 They are to be found of the most various size, and in every tran- 

 sitional stage, from some in which the granular protoplasmic body, 

 besides the vacuole, contains only a single oblong nucleus, which is 

 somewhat pushed to one side, to other large elements in which the 

 protoplasmic body is spread out to form a thin granular mantle, in 

 which at one point a constricted nucleus lies, or two nuclei, or even a 

 number of nuclei at equal distances from each other. In the latter 

 case, the wall appears on optical section to consist of spindle cells 

 fused together by their extremities. Finally, when these interme- 

 diate forms have been observed, we can easily understand the large 

 spherical vesicles, whose wall is distinctly differentiated into separate 

 endothelial plates. Further, it admits of no doubt that while a cell 

 grows into such an endothelial vesicle, buds grow out from the proto- 

 plasm of the cell towards its interior, i.e. towards the vacuole, which 

 become separated off as young cells. In general, such endothelial vesi- 

 cles are formed from lumps which bud from the lymph-canalicular cells, 

 but in many cases also directly from the lymph-canalicular cells them- 

 selves. In the latter case, we can still recognise, even in those elements 

 which have already become transformed into vesicles of considerable 

 size, processes through which the protoplasmic mantle is connected with 

 the neighbouring lymph-canalicular cells. We have, therefore, here 

 before us, in optima forma, the conversion of lymph-canalicular cells 

 into endothelial vesicles, quite on the same principle which was shown 

 by me to obtain in the developing blood-vessels of the embryo chick. 



