528 DEVELOPMENT OF BLOOD-VESSELS. [SECT. 222. 



their walls (the innermost cells excepted) commence to elongate 

 into fibres, and to represent the different fibrous tissues and coats. 

 At the same time, the vessels become thicker ; at first, perhaps, 

 from an increase in number of their cells, by a deposition of new 

 cells from the surrounding blastema rather than by an independent 

 multiplication of them ; subsequently, however, the increase in the 

 thickness of the vessels is effected chiefly, if not wholly, by the 

 growth of their elements in length and in thickness. — In the other 

 type, which has hitherto been little studied, the larger vessels are 

 developed by metamorphoses of capillaries; cells being deposited 

 externally upon them, and being gradually transformed into the 

 different fibrous tissues of the arteries and veins. According to my 

 observations, this mode of development is much more widely dis- 

 tributed than the former ; and, at any rate, it is the manner in 

 which all the larger vessels are formed in the organs, after the latter 

 have once left their rudimentary condition. In the fifth month of 

 intra-uterine life, the coats and tissues of all the larger and middle- 

 sized vessels may be traced, and it is impossible to perceive a trace 

 of formative cells ; but, on the other hand, the tissues appear far 

 from being fully developed, the muscular fibres are short and 

 delicate, and instead of thick, elastic, fibrous networks, there are 

 only fibrils of the finer kinds ; while in place of the elastic mem- 

 branes themselves, we find only layers of fusiform cells, more or 

 less coalesced. In many vessels at this period, only the longi- 

 tudinal fibrous coat can yet be demonstrated, appearing as a homo- 

 geneous, elastic membrane, close beneath the epithelium ; yet even 

 this is absent in the smaller vessels, and in its stead is found a layer 

 of elongated cells, from which it, in its turn, appears to be formed. 

 It is believed that similar cells are occasionally seen in the adult 

 also, in those vessels where even the elastic inner coat is absent. — 

 The muscular fibres of the heart arise as in other situations from 

 coalescing cells, only that here the cells become stellate, and unite 

 with processes from neighbouring cells to form the anastomoses 

 which characterise the later development of these striated fibres. 



The mode of formation of the capillaries is entirely different 

 from that of the larger vessels, consisting, as Schwann and myself 

 have shown, in a coalescence of simple cells. On the first origin 

 of these vessels, the larger kind of capillaries are formed before the 

 others, by the deposition, in linear series, of roundish, angular 

 cells, which coalesce with each other by the absorption of the par- 

 titions and the contents; while the nuclei remain lying on the 

 cell-membrane, which has now become the coat of the capillary 



