34 STRUCTURAL COMPOSITION OF HUMAN BODY. 



is often effected by a division or multiplication of nuclei 

 in the cavity of a parent cell ; as in the primary blood- 

 cells of the embryo, in the germinal vesicle, and others. 



When cells extend and coalesce, so that their walls form 

 tubes or sheaths, the nuclei commonly remain attached to 

 the inner surface of the wall. Thus they are seen im- 

 bedded in the walls of the minutest capillary blood-vessels 

 of, for example, the retina and brain ; in the sarcolemma 

 of transversely striated muscular fibres ; and in minute 

 gland-tubes. 



Nuclei are most commonly oval or round, and do not 

 generally conform themselves to the diverse shapes which 

 the cells assume ; they are altogether less variable ele- 

 ments, even in regard to size, than the cells are ; of which 

 fact one may see a good example in the uniformity of the 

 nuclei in cells so multiform as those of epithelium. But 

 sometimes they appear to be developed into filaments, 

 elongating themselves and becoming solid, and uniting 

 end to end for greater length, or by lateral branches to 

 form a network. So, according to Henle, are formed the 

 filaments of the striated and fenestrated coats of arteries ; 

 and, according to Beale, the so-called connective tissue 

 corpuscles are to be considered branched nuclei, formed of 

 protoplasm or germinal matter. 



3. Cells. The word "cell" of course implies strictly a 

 hollow body, and the term was a sufficiently good one 

 when all so-called cells were considered to be small bags 

 with a membranous envelope, and more or less liquid 

 contents. Many bodies, however, which are still called 

 cells do not answer to this description, and the term there- 

 fore, if taken in its literal signification, is very apt to lead 

 astray, and, indeed, very frequently does so. It is too 

 widely used, however, to be given up, at least for the 

 present, and we must therefore consider the term to indi- 

 cate, ist, and most commonly, a membranous bag with 

 more or less liquid contents, and almost always a nucleus ; 



