992 SPECIAL PHYSIOLOGY. 



Sometimes more than one white tubular fibre has been seen forming in a 

 single embryonic, or less developed one a fact which would show that the 

 tubular membranous sheath might be developed otherwise than by the cell- 

 walls of coalesced formative cells. 



Bloodvessels. 



The arteries and veins, excepting the very finest, are, as already mentioned, 

 not so much tissues as compound structures, built up of several tissues. They 

 are developed in two very different ways. 



In the first place, the principal vascular trunks, or the arteries and veins of 

 the germinal membrane of the embryo generally, and of its commencing organs, 

 and indeed the heart itself, appear primitively as solid cords, composed of mul- 

 titudes of embryonic nucleated cells. After a time, the innermost part or axis 

 of these cords becomes changed into blood, the soft spaces coalescing and form- 

 ing a system of canals ; whilst the outermost cells are then gradually meta- 

 morphosed, in the ordinary manner, into the epithelial, elastic, muscular, and 

 connective tissues which compose the coats of the vessels. This mode of for- 

 mation is apparently limited to the early and principal vessels ; for subsequently 

 the arteries and veins, which are continuously being added, as the body grows, 

 are developed in another manner viz., by the transformation! of previously 

 constructed large-sized capillaries, the calibre of which is increased, whilst the 

 coats are gradually thickened, by the formation of additional tissues developed 

 in the ordinary way. 



The capillary vessels originate in two modes, according to their size. The 

 larger capillaries are formed by the coalescence of linear series of nucleated 

 cells, and the subsequent absorption of their attached ends, so that a homoge- 

 neous tube is produced, recently shown to be lined with a fusiform epithelium, 

 the nuclei of which seem to be attached to the walls of the vessels. These soon 

 become connected with previously existing vessels, and the blood then enters 

 them. The finer vessels, or those of the actual capillary networks, originate in 

 nucleated formative cells, lying amongst the elements of a newly growing 

 tissue ; these become ramified or stellate, by sending out fine processes or 

 branches, which run towards, meet, and coalesce with, other fine processes 

 growing from the larger capillaries just described ; afterwards they coalesce 

 with processes of other ramifying cells which appear in succession. These 

 coalesced processes and the cells themselves become progressively enlarged 

 and hollowed out, so that a tubular or vascular network is produced, the com- 

 ponent vessels of which, though, at first, so fine as to convey only the liquor 

 sanguinis or plasma of the blood, become ultimately wide enough to carry the 

 blood corpuscles also (Fig. 122, h). New capillaries may also be developed 

 within the meshes formed by the older ones. The walls of the coalesced ram- 

 ified cells constitute the homogeneous membrane of the coats of the capillaries, 

 in which the nuclei of the formative cells, and especially those of the epithelial 

 lining afterwards formed, can be recognized. The more numerous and closely 

 set the stellate formative cells, the closer is the capillary network developed 

 from them. 



Blood. 



This important fluid is primitively developed, as already mentioned, in the 

 interior of the newly forming heart and principal vascular trunks. At first, 

 its structural elements the blood-cells or corpuscles, are colorless cells with 

 faintly granular contents and a distinct nucleus, in all respects identical with 

 the embryonic cells . They soon become loosened, and then separated from each 

 other, by the formation of an intermediate fluid plasma, the new liquor san- 

 guinis; their contents become less granular, and colored by the formation of 

 coloring matter in their interior, but their nucleus remains. They are now 

 red blood-corpuscles ; but as compared with those of the fully formed blood, 

 they are much larger, spherical instead of discoid, darker in color, and nucleated, 

 instead of being destitute of a nucleus. Once formed, they speedily enlarge, 

 elongate, or assume a somewhat flattened and elliptical figure, somewhat re- 



