310 H. E. JORDAN AND J. B. BANKS 



probably underlies the formation of intercalated discs in cardiac 

 muscle, presumably as the effect of strains to which neither 

 smooth nor skeletal muscles are subjected, at least not at corre- 

 spondingly early stages. 



The close essential resemblance between discs of adult Limulus 

 heart muscle (fig. 41), and of fetal mammalian heart and the 

 simpler types of vertebrate hearts in general, is striking and 

 significant. In the Limulus heart the disc is clearly a modifi- 

 cation of the myofibrils about a level bisected by a telophragma. 

 The structure of the intercalated discs in the Limulus muscle 

 (9 and 11), their relation to telophragmata, resemblance to con- 

 traction bands, and their relative scarcity, seem to permit of no 

 interpretation other than one in terms of a modified contraction 

 band. This being so in Limulus myocardium, and also in hearts 

 of lower vertebrates (e.g., teleost fish and amphibia; Jordan and 

 Steele (14) ), the conclusion seems to follow logically that a very 

 similar structure in the fetal mammalian heart has a similar 

 origin, and that its later condition must be explained in terms 

 of further additions and modifications. 



It was formerly thought that in the mammalian heart inter- 

 calated discs did not appear until some time after birth. Jor- 

 dan and Steele first described their occurrence in the heart of 

 pre-natal life in the case of the guinea-pig. Here they were 

 described as first appearing during the last week of the gesta- 

 tion period. Jordan and Steele (14) had studied also earlier 

 fetal hearts but were unable to identify the beginnings of discs. 

 It may be that discs actually did not occur earlier in the guinea- 

 pig heait. Or it may be that on account of the relatively finer 

 structural features they were not discernible. But our experi- 

 ence with the beef heart leads us to surmise that the reason for 

 the failure to identify earlier the discs in the guinea-pig fetus 

 lay in an unsatisfactory staining. 



In looking for discs in the fetal beel-heart we studied first the 

 four-month heart. Discs were not at first clearly identified 

 though there seemed to be some vague and uncertain evidence of 

 their presence. We then proceeded to a study of the seven- 

 month heart. But meanwhile we had prepared tissue also from 



