254 



SPECIAL HISTOLOGY. 



From this period, both textures acquire more and more of the configu- 

 ration 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 elon- 

 gated bands (Fig. 105) 0-001 of a line broad, Avith 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 com- 

 parative 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 of a line, whilst others do not exceed 0-0016, and 

 0-002 of a line. The larger ones are, still, always flattened, but of 

 uniform Avidth, and also considerably thicker than before, mostly with 

 evident longitudinal and transverse strioe, and even Avith fibrils, which 

 admit of being isolated. It is partially evident, even in a longitudinal 



view, but still better in a trans- 

 verse section, that in many 

 cases, the fibrils do not occupy 

 the entire thickness of the pri- 

 mitive tube, but that they are 

 deposited around its j9erz)>/!er?/, 

 the interior being as yet filled 

 with a homogeneous substance 

 as at first, and Avhich now ap- 

 pears like a canal within the 

 fibrils. All the primitive tu- 

 bules possess a sarcolemma {b), 

 Avhich on the application of 

 acetic acid or soda, appears as 

 a very delicate membrane, Avhich 

 by the imbibition of water, may 

 occasionally be raised from the 

 fibrils. The tubes, moreover. 



Fi?. 106. 



Fig. 105. — Primitive fasciculi of an eight to nine weeks' human embryo; magnified 350 

 diameters: 1, two fibres witliout transverse striajj 2, fibres presenting the first indications 

 of transverse striation ; a, nuclei. 



Fig. 106. — Primitive fibres of a four months' huinan embryo; magnified 350 diameters: 

 1, a fasciculus, with a clear, as yet, non-fibrillated substance in the interior: 2, fasciculus 

 without such contents, with an indication of transverse striation ; o, nuclei ; b, sarcolemma. 



