TISSUES OF THE BODY. 



385 



A cell of this kind, then, swells up into a vesicle, while at the same 

 time its nucleus undergoes segmentation. Some of these nuclei presenting 

 great regularity of arrangement throughout the protoplasm cortex (endo- 

 thelium vesicle of Klein), the latter may be regarded as made up of not yet 

 separated endothelial cells. These are to be seen later more distinctly. 



From the endothelial wall further yellow-coloured and white cells spring, 

 by a process of gemmation, 

 the first blood corpuscles. 

 The genesis of these cells, 

 therefore, appears now un- 

 der a new light (81). 



In other of these forma- 

 tive cells the inner portion 

 of the protoplasm is said to 

 assume a red colour, and to 

 divide itself over the new 

 nuclei formed by segmen- 

 tation to form blood-cor- 

 puscles. Finally, coarsely 

 granular cells are said to 

 undergo a precisely similar 

 process of transformation. 



Thus, we see that both 

 the walls of the vessels and 

 the first blood corpuscles 

 take their origin from the 

 same cells, "the brood- 

 cells" (Brutzellen) of Klein. 



But how is the vascular 

 tube formed from these sepa- 

 rate endothelial vesicles'? 



The first of the vesicles 

 become elongated and sac- 

 culated. They may, how- 

 ever, only at first send out 

 laterally solid buds of proto- 

 plasm which subsequently 

 become hollowed. Now, 

 by the union of all these, 

 one with another, the first vascular tubes are formed. 



Even the largest vessels and the heart itself appear to have a similar origin. 



We have before us, then, protoplasm tubes which gradually divide into 

 endothelial cells by a process of segmentation of the nuclei. This agrees 

 very perfectly with an old-established fact, namely, that only from a 

 certain stage of development on, does the nitrate of silver solution pro- 

 duce the well-known mosaic appearance in the walls of capillaries. 



Very early it may be remarked in the chick that the growing arteries 

 receive a clothing of flat stellate cells forming an embryonic adventitia. 



The further development of vessels, but more especially of new capil- 

 laries from those already present, agrees most beautifully with all this. 

 On this point we have lately had some very excellent observations by 

 Arnold, with some not quite so recent. 



An object formerly much studied has recently received much attention. 



Fig. 384. Development of fine capillaries in the tail of the 

 tadpole. />, p, protoplasm sprouts and cords. 



