TISSUES OF THE BODY. 301 



172. 



Turning now to the development of the tissue, we find smooth muscle to 

 take its rise from the simple transformation of round formative cells with 

 spheroidal vesicular nuclei of the middle germinal plate. These elements 

 change into contractile fibre-cells by growth in two opposite directions, 

 during which the nucleus assumes its before-mentioned elongated con- 

 figuration (p. 281). Fig. 273 (a, b) represents two such embryonic cells 

 from the wall of the stomach of a foetal pig two inches long. 



Touching now the striated structures, it was for a long time supposed, 

 in accordance with Schwann's view, that the fibre was always produced by 

 a fusion of formative cells arranged in rows, whose united membranes 

 went to form the sarcolemma, while the nuclei persisted, and the com- 

 bined contents of the cells took on the characteristic form of fleshy matter 

 through further metamorphosis. 



But this view is, as we now know for certain, quite erroneous. The 

 muscular fibre, far from being a result of the fusion of a series of cells, is 

 nothing more than a single elongated filiform cell, in which the nucleus 

 has undergone division and multiplication, and the contents metamor- 

 phosis, and which has attained gigantic dimensions in proportion to the 

 length of the striped muscle. We have already referred to this mode of 

 development (for the discovery of which we are indebted to Lcbert and 

 Remak] in discussing the growth of the tadpole (p. 96). 



In the mammalia and human beings the same is to be observed. Here 

 we may follow up, in young embryos, the steps in development of the 

 tissue, which are essentially similar. 



Thus in the human foetus, at about the sixth or eighth week, very 

 narrow membraneless and fusiform cells, often only '0025-0*0036 mm. 

 in breadth, are met with as elements of the rudimentary muscle of the 

 hands and feet. They are formed of very delicate protoplasm, with a 

 single or double vesicular nucleus, and attain a length of 0'14 0*18 mm. 

 (Koelliker, Frey). 



The same is to be seen in mammalian embryos at corresponding stages 

 of development. In those of the sheep, measuring 279 mm. in length 

 (fig. 293), we may obtain from the diaphragm and abdominal muscles 

 fusiform cells 0'28-0'38 mm. long, and 0'0045-0'068 mm. in breadth. 

 These show a vesicular nucleus 0'0077-0'0104 mm. in diameter, and 

 incipient transverse striation in the central portion (a, b). These nuclei 

 range in number from two to four, but other cells further advanced 

 possess many more of the latter (c), and increase in transverse diameter to 

 double or even more (d). As a rule, their axis remains unaffected by the 

 transverse striation, and in it we see the original protoplasm. In some- 

 what older animals the muscular fibre is "01 29-0 '01 56 mm. in thickness, 

 and so long that it can no longer be isolated in its entire length, although 

 the pointing of one end (e). or blunt rounding off of the same (/), may 

 be easily found. The number of nuclei now becomes greater and 

 greater, arid the process of division is observed as an ordinary occurrence 

 (e,f y g). Sometimes the position of the former is central (/, g, i) and 

 sometimes peripheral (//). The axis of the fibre generally remains free 

 from transverse marking (/, h, g), while at its circumference the longitu- 

 dinal cleavage commences to manifest itself. The tendency among muscles 

 of this kind to break up into thick discs under the action of water (i) is 

 a point of much interest. 



