THE BLOOD-VESSELS. 



99 



feet accessories of an earlier epoch. They are rarely used 

 at present. Other vehicles for the coloring material, resin- 

 ous, waxy masses, or etherial oils, are, at the most, only here 

 and there employed for very special purposes. 



The embryonic origin of the vessels is still attended with 

 many obscurities. 



The heart, a production of the middle germinal layer, is 

 formed very early, and enters soon afterwards into activity. 

 It is hollow from the commencement, and the large, adjacent 

 blood-vessels likewise appear to possess the same charac- 

 teristic. 



With regard to the more particular details of this process, 

 we must state that our present knowledge is little satisfactory. 



According to Klein, the first large vessels of the hen's em- 

 bryo are formed from cells of the middle germinal layer. 

 The contents of the latter soon liquefy. A protoplasma shell 

 now invests the enlarged and macerated cell-body with the 

 original nucleus. From such cells are derived the first vascu- 

 lar wall, or endothelial tube, as well as the first blood corpus- 

 cles. The cell is said to swell, and the nuclei increase, and, 

 as during this increase the nuclei assume a regular position, 

 the protoplasma mantle finally divides into flat, endothelial 

 cells. From these endothelial walls the first blood corpuscles 

 are also said to take their origin by a process of constriction. 

 They are said, however, to have another origin, also. 



The first vascular walls and the first blood corpuscles, 

 therefore, derive their origin from the same cells. 



We add to this the important fact that it is only subse- 

 quently to the use of nitrate of silver solution that the pri- 

 mary vascular wall is resolved into the familiar endothelial 

 cells. 



A process of aggregation then leads, in a secondary man- 

 ner, to the formation of the additional external vascular 

 layers, a serosa, media, and adventitia. There is here, also, 

 a great want of accurate observations. 



Capillaries — we assume, at first, a homogeneous, nucleated 

 protoplasma tube — are present at an early period. 



