HISTOLOGY OF THE FROG 97 



sarcomere becomes bulged out laterally, so that the whole 

 sarcostyle has a moniliform appearance. It is held that the 

 sarcous elements are composed of a denser protoplasmic 

 substance, spongioplasm, and that the clear areas are occupied 

 by a more fluid substance, the hyaloplasm, and that in the 

 process of contraction the more fluid hyaloplasm is forced into 

 or flows into the pores which permeate the spongioplasm of 

 the sarcous elements. This, at least, is one explanation of the 

 structure of striated muscular fibres, but it is held by other 

 authors that a muscle-fibre is composed of a very regular intra- 

 cellular network of modified highly contractile protoplasm, 

 holding in its meshes a more fluid hyaloplasm, and that the 

 optical effect of cross-striation is due to the regular arrange- 

 ment of the network. It would be beyond the scope of this 

 work to enter into a discussion of the relative merits of the 

 rival theories, and it need only be remarked here that the 

 theory of an intra-cellular network accords better with our 

 present knowledge of the structure of protoplasm, than does 

 the elaborate theory of sarcomeres, sarcous elements, etc., 

 divided up by membranes. Both theories resemble one 

 another in assuming the existence of a denser, actively-con- 

 tractile spongioplasm, and a passive, more fluid hyaloplasm. 



Cardiac muscular tissue (fig. 18, B and C) is, in some 

 respects, intermediate between plain and striated muscular 

 fibre. Its cellular components are not fused together, but 

 remain distinct, so that the muscle appears to be made up of 

 a number of oblong nucleated cells, joined end to end to form 

 fibres, and communicating by short processes with the cells of 

 adjacent fibres. Each cell exhibits cross striations not unlike 

 those of striated voluntary fibres, but less distinct. 



Ordinary striped muscle-fibres are formed from somewhat 

 elongated embryonic cells, which are at first simple and 

 uni-nucleate. At the time of embryonic life when muscle is 

 formed each cell elongates and its nucleus multiplies, so that 

 a long multi-nucleate protoplasmic fibre is formed devoid of 

 striae. The last-named first appear as longitudinal fibrils 

 running along one side of the cell, and the whole has now the 

 appearance of the very primitive epithelio-muscular cells found 

 in some of the lower multi-cellular animals. At about this 

 time the sarcolemma is formed as a delicate membrane 

 bounding the fibre. Presently cross as well as longitudinal 



