108 DE. WILSON FOX ON THE DEVELOPIVIENT 



cular fibre as a process which consists in a definite series of changes, commencing in the 

 cells of the early embryo. I have already adduced the reasons which dispose me to 

 retain this term for these very distinct anatomical structures, and in their later stages 

 I regard them as possessing all the essential elements of a cell* (see Plate V. figs. 6-13, 

 from Tadpole). This, however, is a question on which the opinions of different 

 observers seem at present scarcely reconcileable, and therefore it is only possible for 

 me here to state my individual views as to their nature. The next most noticeable 

 feature which is to be observed is the change which takes place in the interior of these 

 bodies, — a portion of their substance undergoing a conversion into a material which 

 has all the recognized visible characters of muscular tissue, while the nucleus continues 

 free and external to the striated portion, the whole being surrounded by a membrane. 

 This membrane I regard as the sarcolemma, as its presence can be traced with high 

 magnifying powers through every gradation of development, from almost the earliest 

 stage, to that of fully-formed muscle ; and I am therefore disposed to regard it as pro- 

 duced by an extension of growth, including an increase in thickness of the outermost 

 layer (whether or not this be termed cell-wall) of the primitive cell. That this mem- 

 brane is not demonstrable in the fibres of the heart at later stages, may, I think, 

 possibly be due to its exceeding tenuity rather than to its absence ; and as the position 

 of the nuclei in relation to the fibre is similar to that found in other muscular tissue, it 

 appears to me probable that the whole structure is held together by some limiting wall ; 

 since it is seen that in all stages of development the nuclei are external to the striated 

 portion, which appears to be formed by a gradual conversion of fresh material accumu- 

 lated during growth within the sarcolemma, and probably under the influence of the 

 nuclei as nutritive centres. These latter seldom or never appear to be imbedded within 

 the fibres, though here and there, in the muscular fibre of the adult Frog, this may occa- 

 sionally be the case. 



It will be seen that my descriptions accord very closely with those of Lebert, Remak, 

 and KoLLiKER. The early stages in the Chick and Tadpole (Plate V. figs. 2-6, & 

 fig. 22) correspond very closely with M. Lebeet's figs. 11, 21, & 26f. After this stage 

 there is, however, some discrepancy between M. Lebekt's and my descriptions, as there 

 is also in the period at which he states the differentiation in the Chick to commence. 

 I have found it most distinctly at the end of forty-eight hours of incubation in the 

 dorsal region; M. Lebeet states that no traces of muscular fibre are formed before 

 the fifth day, — a period at which I have found the process considerably advanced. 

 M. Lebeet's description of the development of the fibres of the heart differs con- 

 siderably from mine. 



My observations correspond closely with those of Professors Remak J and Kollikek §, 

 and I am glad to know that the opinions at which I have arrived agree with those 



* See note, p. 102. t Ann. Sciences Nat. 3rd series, vol. ii. 1849, 



X Froriep's Notizen, 1845 ; and Entwickelung der Wirbelthiere, Taf. xi. 

 § Zeitsch. Wiss. Zool. ix. Gewebelelire, 1862. 



