DEVELOPMENT AND GROWTH. 



generally divides into two, and by further division these are multiplied ; a 

 fine membrane, at first absent or invisible, is soon discovered, bounding the 

 cell and inclosing its contents. In the mean time the substance becomes 

 striated longitudinally at one fart, and more transparent, the granules dis- 

 appearing. The striatiori, which is the first indication 

 of the proper muscular substance, extends throughout pig. LXXIII. 

 the length of the elongated cell, but at first affects 

 only a small part of its breadth, and the remaining 

 space is occupied by unchanged granular matter and 

 the nucleus or nuclei which lie on one side. In due 

 time, however, this conversion into the proper muscular 

 substance, further shown by the appearance of cross 

 strise, proceeds through the whole thickness of the cell, 

 or fibre as it may now be called ; the inclosing cell- 

 membrane becomes the sarcolemma, and the nuclei, 

 with a small residue of the granular protoplasm still 

 adhering to them, remain. 



Growth of muscles. The muscular fibres of the growing 

 foetus, after having acquired their characteristic form 

 and structure, continue to increase in size till the time 

 of birth, and thenceforward up to adult age. In a full 

 grown foetus most of them measure twice, and some of 

 them three or four times their size at the middle of foetal 

 life ; and in the adult they are about five times as large 

 as at birth. This increase in bulk of the individual 

 fibres would, of course, so far account for the con- 

 comitant enlargement of the entire muscles. But there 

 would seem to be also a multiplication of the fibres ; 

 and Budge believes he has proved this as regards the 

 muscles of frogs. Two modes of production of new 

 fibres have been described viz. first, from connective 

 tissue corpuscles lying between the existing fibres, by a 

 process analogous to the original development of the 

 muscle (von V/ittich) ; secondly, by the splitting up 

 of a fibre through its whole length into two or more 

 smaller ones, preceded by multiplication of its included 

 nuclei. This second process has been observed by 

 Weismann and by Kolliker in frogs, in the winter 

 season, and appears to serve for the replacement of fibres 

 destroyed by fatty degeneration, which is said to be 

 not uncommon in these creatures. Dr. Beale, however, 

 denies that the new and slender fibres are derived from 

 an old and larger one by splitting of its substance ; he 

 believes that they are produced from cells, as in the first 

 mode, and that the old fibre is removed. The great 

 increase in the muscular tissue of the uterus during 

 gestation takes place both by elongation and thickening 

 of the pre-existing fibre-cells of which that non- striated 

 tissue consists, and by the development of new muscular 

 fibre- cells from small, nucleated, granular cells lying in 

 the tissue. In the shrinking of the uterus after parturition the fibre-cells 

 also shrink to their previous size ; many of them become filled with fat-granules 

 (fig. LXXIII.), and many are doubtless removed by absorption. 



Fig. LXXIII. 

 MUSCULAR FIBRE- 

 CELLS FKOM THK 

 UTERUS, THREE 

 WEEKS AFTER DE- 

 LIVERY. THE UP- 

 PER FOUR TREATED 



WITH ACETIC ACID, 

 MAGNIFIED 350 



DIAMETERS (from 



Kolliker). 

 a, nuclei ; 7, fat- 

 granules. 



