126 



THE TISSUES 



separated into "cells," especially in young animals and in lower vertebrates, by 

 use of those chemicals usually used to break down intercellular cement; have aU 

 been used as arguments in favor of a cellular structure. On the other hand, the 

 fact that it has not been proved that the segments bounded by the^discs corre- 

 spond to the original myoblasts; that in the later stages of development of heart 

 muscle the type of nuclear division is usually amitotic, a type frequently unac- 

 companied by division of the cytoplasm; that the fibrillae pass uninterrupted 

 through the discs; that some discs only partially cross a fibre; that some segments 

 contain more than one nucleus while others are non-nucleated; all favor a 

 syncytial interpretation. How open the question still remains is shown by the 

 fact that Heidcnhain regards the "cells" as "growth segqients"; that Marceau 

 considers them non-contractile "supports" for the fibrillae; that Jordan from 





M 







* .r'-i'ft "^ v'l i . f--^JSjii» 





Fig. 69. — Longitudinal Section of Heart 

 Muscle of Rabbit (Werner) showing cross and 

 longitudinal striations and intercallated discs. 

 In several "areas" two nuclei are seen. 



Fig. 70. — Longitudinal Sec- 

 tion of Heart Muscle (Przen- 

 oski), showing continuity of fi- 

 brils through intercallated disc. 



his work on humming-birds concludes that they are a coarser development of 

 the anisotropic bands; while Schaffer looks upon them as post-mortem contrac- 

 tion artefacts. 



Development of Muscle Tissue 



Smooth Muscle. — In the higher animals, muscle tissue with the exception of 

 that connected with the sweat, lacrymal and mammary glands, which is of 

 ectodermic origin, is derived wholly from mesoderm. The cells (myoblasts) 

 which are to become smooth muscle cells develop in the general mesenchymal 

 tissue among cells which are to become connective-tissue cells and with which 

 they are at first apparently identical. In becoming a smooth muscle cell the 

 myoblast chariges its shape, becoming greatly elongated, its nucleus at the same 

 time becoming oval or rod shaped. Such cells anastomose freely. During these 



