IN THE DEVELOPMENT OF TISSUES. 127 



epithelial coat of the vessel. In the next layer, which forms the so-called 

 internal coat of the vessel, the nuclei remain unaltered. But in the for- 

 mation of the muscular or contractile coat of arteries, the nuclei elongate 

 and arrange themselves in rows in the manner before described. Moreover 

 each row of elongated nuclei appears to appropriate the adjoining strip of 

 structureless membrane in which it is imbedded, and the result is that this 

 membrane breaks up into a number of flat fibres, each bearing upon its 

 surface the row of nuclei after which it was modelled. Organic muscular 

 fibres of other parts of the body are formed after exactly the same plan. 

 In the formation, also, of fibre-cellular or areolar tissue, the nuclei are 

 arranged in rows, to each of which is appropriated a strip of the cytoblas- 

 tema ; and each such strip, instead of remaining flat and ribbon-like, as is 

 the case in organic muscular fibre, breaks up into a bundle of parallel 

 longitudinal fibrillse. This is quite opposed to the account given by 

 Schwann* of the development of fibro-cellular tissue. Kolliker,f in 

 alluding to these several transformations undergone by the nuclei, men- 

 tions also, as other instances, the different modes of development of 

 seminal filaments directly from nuclei,^ and the growth of the spines of 

 several invertebrate animals. 



Arguments in favour of the view of the importance of the nuclei to 

 the growth and well-being of the tissues in which they occur, are furnished 

 also by the phenomena which attend their retrograde, as well as their 

 advancing transformations their degradation as well as their develop- 

 ment. For it has been rendered highly probable by the investigations of 

 Mr. Paget, that in all cases of atrophy accompanied with degeneration of 

 tissue, the nuclei of the degenerated part lose their characteristic proper- 

 ties, or entirely disappear. This is especially the case in fatty degenera- 

 tion (or atrophy) of muscle, of the liver and of the kidney, in all well-marked 

 instances of which, the nuclei, of the fibres in one case, of the hepatic and 

 renal cells in the other cases, have completely disappeared, their place 

 being occupied with fat, in the form of granular matter, or drops of oil. 



Development of the Blood. 



It may be desirable here to present some account of the principal ob- 

 servations recently made on the development of the blood corpuscles. 

 Concerning the original formation of these corpuscles in the embryo, the 

 results of nearly all recent investigations tend to shew that, as was stated 

 by Reichert,|| at the first appearance of a vascular system they consist, 

 in all vertebrate animals, of nucleated, colourless, granulated cells, identi- 



* Mutter's Physiology, p. 1646, and fig. 253. 



f* Entwickelungs-gesch. der Cephalopoden, p. 145. 



J For an account of these modes of development, see p. 41. 



$ Lectures at the Royal College of Surgeons, May, 1847. Lecture V. 



II Mullet's Physiology, p. 1550. 



