DEPARTMENT OF EMBRYOLOGY. 109 



which this substance develops depends upon the treatment to which 

 it has been subjected. Thus, she was able to demonstrate complete 

 cross-striations in all the stages studied, up to 4 days, and that fibrils 

 are completely cross-striated from their first appearance — i.e., at the 

 stage of 10 myotomes, at which time it has heretofore been supposed 

 that there existed only threads and granules from which the fibrils are 

 formed. It is difficult to distinguish the heart-muscle before 10 myo- 

 tomes when the heart forms a single tube composed of an outer layer 

 of flattened cells and an inner layer of endothehal cells. In the very 

 young embryos the muscle-tissue appeared to be in the form of a 

 syncytium; only a few fibrils were formed at this time, but as the heart 

 matured the number increased. In all ages they varied in wddth, from 

 that of a mitochondrium to the full diameter of the cell. They were 

 usuallj^ straight or shghtly bent, but in some places curved with the 

 outline of the cell. The wdng-muscle of the house-fly and the heart- 

 muscle of the chick, examined side by side, showed marked similarities 

 in their cross-striations. These striations are designated by Mrs. 

 Lewis as the dark hand, the light hand, and the gray hand, and are formed 

 respectively of Krause's membrane, hyaline substance, and sarcous 

 substance. The most pronounced was the fight band, which is fighter 

 than the cytoplasm, for the reason that it does not become as dense 

 upon fixation as other portions of the cell. The dark band is probably 

 quite dense, while the gray band is practically the same as the cyto- 

 plasm. The gray band was not noticeable as a special structure, but 

 the regular space between the two light bands indicated its presence. 

 The characteristic appearance of the bands remained practicaUy the 

 same wherever found, and whether wide or narrow. In certain round 

 cells the fibrils could not be found, but the bands were present, repre- 

 sented at times by granules of uniform size surrounded by a light area. 

 For the study of li\dng muscle the same method was used, except that 

 a drop of Locke solution was used in the place of the fixing solution. It 

 was thus possible to study the living heart, which continued to con- 

 tract for an hour or more. In such preparations no structures resem- 

 bling the fibrils in the fixed preparations were present in any of the 

 stages studied, not even in 6-day embryos which were used for com- 

 parison. There were, in fact, no long threads of any kind. The mito- 

 chondria were abundant, as shown by the treatment with Janus green, 

 and, although in the form of threads and rods, they did not extend far 

 beyond the region of the nucleus. Cross-striations are present in 

 embry^os of 10 myotomes, but without careful search are difficult to 

 observe up to 2 days of incubation. The most active cells contain 

 them as parallel bands of fight and dark material which are lost as 

 soon as the vision penetrates below the surface of the cell. The cross- 

 striations are, however, readily observable in the living heart when 

 stained with Janus green, although in no instance were they as marked 



