272 SPECIAL HISTOLOGY. 



§ 86. 



Development of the Muscles and Tendons. — The rudiments 

 of the muscles consist, originally, of the same formative cells as 

 those of which the rest of the body of the embryo is con- 

 stituted; and it is not till afterwards that the muscles, tendons, 

 &c, are gradually developed by a histological differentiation. 

 In man, the muscles are not evident before the end of the 

 second month ; at first, however, they cannot be detected 

 by the unaided eye ; they are soft, pale, gelatinous, and not to 

 be distinguished from their tendons. In the tenth and twelfth 

 week they are more distinct, especially in specimens preserved 

 in alcohol ; and at this time the tendons also may be distin- 

 guished as somewhat clearer, but at the same time trans- 

 parent streaks. 



In the fourth month, both the muscles and tendons are 

 still more distinct, the former being, on the trunk of a light 

 reddish colour, the latter less transparent, and greyish, both 

 retaining a soft consistence. From this period, both textures 

 acquire more and more of the configuration which they 

 afterwards retain, so that at the maturity of the embryo, 

 — excepting that the muscles are still softer and paler, and the 

 tendons more vascular and less white, — they no longer present 

 any difference worth notice. 



With respect to their intimate conditions, the primitive 

 fasciculi, in the embryo, at the end of the second month, present 

 the aspect of elongated bands (fig. 105) 0-001"' to 0-002'" 

 broad, with nodular enlargements at different points, at which 

 places are situated elongated nuclei ; the bands exhibit either 

 a homogeneous or finely-granular aspect, and but rarely an 

 extremely faint indication of transverse striation. In their 

 further development, these primitive muscular fasciculi, which, 

 as comparative histology teaches, originate in cells arranged in 

 a linear series, continue to increase in breadth and length, and 

 their contents, the original cell-contents, are developed into the 

 muscular fibrils. In the fourth month (fig. 106) they measure 

 for the most part 0-0028 — 0-005'", some even 0-006'", whilst 

 others do not exceed 0-0016'" and 0-002'". The larger ones 

 are, still, always flattened, but of uniform width, and also 

 considerably thicker than before, mostly with evident longi- 



