INTERCALATED DISCS OF HEART MUSCLE 165 



they extend across two fibers, and very frequently the discs are 

 at the same level in adjacent fibers. They are always superficial, 

 never appearing in the mid-line of a fiber when the nucleus is in 

 focus. They appear in patches, and without regard to the posi- 

 tion of the nuclei, and are only very rarely in 'steps.' 



B. YOUNG AND FOETAL MATERIAL 



1. Guinea-pig 



a. Second, third and fourth weeks. In young guinea-pigs of the 

 second to fourth week intercalated discs are already present 

 (figs. 20 and 21). The majority, however, are in the shape of 

 very narrow bands. These may be arranged in ' steps' connected 

 by 'risers.' Many of the apparently compact discs are resolved 

 under higher magnification into a series of blocks (local thicken- 

 ings of fibrils), and the discs in general are less compact than at 

 older stages. Again there is absolutely no evidence of cells and 

 boundaries. 



b. First week. During the first week discs are already present, 

 and at the usual levels, but all have a light-staining character. 

 In this tissue the granular discs shade gradually laterally into 

 the darker bands. 



c. Last week of gestation. During the last week of gestation, 

 coincident with the appearance of striations, discs first make 

 their appearance (fig. 23). No indication of discs could be seen at 

 any earlier period. The discs here are narrow and consist of 

 faintly-staining granules. 



2. Cat of four days 



In a cat embryo of four days the discs are already sparsely 

 present. They are evidently just making their appearance. 

 They are narrow, pale and granular. 



3. Child of four years 



In this tissue the discs are abundant, and mostly of the type of 

 narrow plates of slight depth. Relative to their number in the 



