or STRIATED MUSCULAE FIBEE. 105 



represented in Plate V. figs. 3, 5. In one instance, represented in Plate V. fig. 20, I 

 found a fibre dividing, and I could distinctly trace a delicate membrane outside the 

 nuclei which lay on the fibre contained on each of the divisions*. 



As development advances, the amount of space occupied relatively by the granular 

 portion of the contents of the membranous envelope diminishes, and its place becomes 

 gradually occupied by striated matter. A membrane may still be traced over the 

 whole structure ; and in all the instances which I examined, the nuclei were situated 

 between the membrane and the striated portion, surrounded by a little dimly granular 

 material. Plate V. fig. 21 represents this condition from the upper part of the tail of a 

 Tadpole one week old, when, though seen in profile, it will be observed that the structure 

 has all the characteristics of adult muscular fibre. 



The investigation of the earlier stages of muscle in the Chick presents much greater 

 difficulties than in the Tadpole, owing both to the smaller size, and also to the extreme 

 delicacy of the structures concerned. 



The first difierentiation from the round formative cells of the embryo which I have 

 observed commences from the second day of incubation. At this period there are found 

 in the dorsal region oval bodies such as are depicted in Plate V. figs. 22, 23, fig. 22 mea. 

 suring tiVo of an inch in length by -^^tr of an inch in breadth, and containing a nucleus 

 (in which a nucleolus is often but not invariably seen) of g^^o X tooo of an inch. 



These bodies are dimly nebulous ; they have a distinct, clear, well-defined outlme, but 

 they cannot be seen to be surrounded by a membrane. The nucleus is often granular, 

 and its outline is particularly well defined. 



Almost simultaneously (^. e. after forty-eight hours of incubation), appears a series of 

 forms like figs. 23, 24, 25 (Plate V.) — bodies tending to become fusiform, and of which 

 I believe fig. 23 to represent the earliest stage. These also have in most cases a well- 

 defined outline, and contain a large nucleus which is particularly well defined (Plate V. 

 figs. 24 & 25 represent the appearances most commonly seen at this date, fig. 25 being a 

 similar structure to fig. 24, but seen in profile). For the same reasons as 1 stated with 

 regard to similar structures in the Tadpole, I look upon these bodies as cells ; and the 

 impossibility of distinguishing any isolable membrane arises, I believe, only from its 

 excessive tenuity. They gradually tend to become much elongated and tapering at 

 their extremities, and at this stage form a continuous tissue (as seen at Plate V. fig. 

 26), in which, though these bodies lie in close apposition, the outlines of each are 

 distinctly maintained. In some of these bodies at this stage, two nuclei are seen (Plate 

 V. fig. 27), but this does not appear to be the rule in the Chick. 



Even at this early stage there may sometimes be seen a faint striation of portions of 

 the cell-contents, usually appearing first in the form of longitudinal striae (as seen in 

 Plate V. fig. 25-27, figured in profile). 



By the commencement of the third day of incubation a further change has appeared in 



* KciLLiKER has also seen a division of the fibres in the tail of the Tadpole, but does not mention whether he 

 obseiTcd the membrane thus disposed (Gewebelehre, p. 102). 



