QUARTERLY CHRONICLE OF MICROSCOPICAL SCIENCE. 183 



vesicles, whose wall is the original body of the cell. From 

 the nucleus, which is at first simple and clear, several oblong 

 nuclei arise, which separate more and more from one another 

 in proportion as the vacuole increases, till they are ultimately 

 arranged at pretty regular intervals. The wall of the cell at 

 this stage is compared by Klein to an endothelium, and the 

 vesicles he calls " endothelial vesicles." Cells which are 

 some of them coloured, some white, are separated from the 

 protoplasmic wall of the vesicles (which is the endothelium 

 of the futui-e blood-vessel) and are contained in the interior 

 of the vesicles. 



(b) The substance of some other cells shows a yellow tinge 

 round the one or two centrally situated nuclei ; an<l later 

 on the central part of the cell substance distinctly splits up 

 around the nuclei, so that none but coloured nucleated 

 blood-corpuscles are contained in the interior of the cell. 



(c) Certain coarsely granular formative elements become 

 altered in such a Avay that when the coarse granular appear- 

 ance is gone and a multitude of nuclei have come into view 

 the central part of the protoplasmic substance becomes 

 yellowish, and becomes marked off as blood-cell substance, 

 while the peripheric part of tliese elements represents the 

 protoplasmic wall, the endothelium of the future vessel. 



In all these three cases blood-corpuscles arise by an 

 endogenous process from the central ])art of certain cells of 

 the middle germinal layer, while at the same time a proto- 

 plasmic and regularly nucleated wall^ (endothelial wall) is 

 formed out of the peripheral part of the same cells. 



It is also shown how a closed tubular system, the vascular 

 system, arises out of these germinal cells containing blood- 

 corpuscles. The vesicles which are at first round grow out 

 into an elongated shape, or swell out in various directions as 

 they grow, while at the same time they come considerably 

 nearer to one another. The wall of these vesicles often sends 

 out solid threads and shoots which afterwards become hol- 

 lowed out, and into the cavity of which the cavity of the cells 

 containing blood-corpuscles is continued. In this way there 

 arise irregular branched and vascular structures which become 

 transformed into a continuous system of tubes by the con- 

 necting threads which pass from one blood-cell to another 

 becoming hollowed out. In this way results a system of 

 tubes forming a network, and filled though not uniformly 

 with blood-corpucles. The cavity of the heart, the aorta, and 

 the sinus terminalis, arise according to Klein's observations 

 in the same Avay. He finds that the earliest blood-vessels 

 both of the area opaca and the area pellucida, are developed 



