384 MANUAL OF HISTOLOGY. 



211. 



As to the development of the vascular system, we know that it takes 

 place from the middle embryonic plate. According to an old and wide- 

 spread theory, the heart and the earliest trunks which appear in the 

 rudiments of the embryo, namely, the arch of the aorta and primitive 

 veins, are met with at first in the form of solid cylinders composed of 

 cells, without any distinction between the axis and peripheral portion. 

 The latter, through the close union of its cells, becomes the primary wall 

 of the vessel, while the cellular elements in the axis constitute, on the 

 liquifaction of the intercellular substance, the first blood corpuscles ( 81). 

 More recent investigation, however, has shown that the heart is developed 

 from the very commencement as a hollow organ (Sclienck, Hensen, Klein). 

 In the embryonic chick, Remak states that he has recognised the first blood- 

 vessels in the form of solid cylinders 0'0282-0'0451 mm. in breadth; in 

 the transverse section of which, as a rule, from three to eight formative 

 cells may be seen, but at times only two. At a more advanced stage of 

 development these cylinders are observed to be hollow and tubular, their 

 walls consisting of a single layer of formative cells projecting far into the 

 interior, and constituting, in all probability, the endothelium of a later 

 period. Subsequently great stress was laid here also upon the hollow 

 rudiment. 



The vessels of a later period were long supposed to be developed after 

 another fashion, namely, from the fusion of simple rows of cells with sub- 

 sequent investment in other cells. 



This is almost identical with the mode of development which, since 

 the days of Schwann, has been regarded as that of the capillaries. 



The latter, as the older theory goes, are formed by the melting down 

 of the central portion of formative cells, which, arranged in rows one after 

 the other, and united in this position, are converted into a tube by open- 

 ing into one another. In this case the membrane of the cell becomes the 

 wall of the vessel, and the nucleus remains with it, as is always seen. 



The formation of non-branching capillary tubes was believed to occur 

 in the following manner : It was supposed that fusiform cells, arranged 

 one after the other, became united by means of their processes, the differ- 

 ence in the diameter of the cell, and its ramifications, gradually disappear- 

 ing later. The cellular tubes so formed were then supposed to receive 

 their blood by becoming connected with the previously existing vessels. 



But owing to the fact that such unbranched tubes but seldom occur in any 

 length, and that a retiform arangement of tubules is the rule, the stellate 

 cell was regarded as playing an important part in the production of ramifica- 

 tions among rudimentary capillaries. This also has since been shown to 

 be incorrect. The lumen of the capillary vessel is an intercellular space. 



Seeing, then, that these older views are no longer tenable, let us inquire 

 what have recent observations done towards showing the true mode of 

 development of the blood-vessels. 



Let us take first that of the earliest rudimentary vessel in the foetus, 

 which is of considerable size, as is well known. 



The first-formed vessels in the chick spring, according to Klein, from 

 cells of the middle germinal layer, whose contents become fluid until the 

 enlarged and watery cell-body is contained within a cortex of protoplasm 

 supplied with a nucleus. From such cells the primary wall of the vessel 

 takes its origin with the endothelium tube and first blood-corpuscles. 



