282 Papers from the Department of Marine Biology. 



was to be expected that intercalated disks should occur in a heart- 

 muscle essentially so much like that of vertebrate cardiac muscle, with 

 a similar rhythmic beat; and considering the low grade, and the slow 

 rate of heart-beat (32 beats per second, Patten) of the organism, they 

 were expected only in small number and of very simple form. These 

 expectations are realized. However, the extreme rarity of the disks 

 is surprising in view of the fact that they actually occur. 



LITERATURE. 



The only work known to me on the microscopic structure of Limulus 

 muscle is that by Meek (1909) on the cardiac muscle. He describes 

 its structure as a double syncytium composed of trabeculse "individ- 

 ualized by connective-tissue sheaths"; the peripheral nuclei of the 

 trabeculaB he interprets as connective-tissue nuclei. He records also the 

 absence of sarcolemma and of intercalated disks, and notes the close 

 similarity between the heart-musculature of Limulus and that of lower 

 vertebrates. Carlson (1904) demonstrated the applicability of the 

 neurogenic theory of heart-beat to Limulus. Meek concludes that a 

 syncytial heart-musculature, accordingly, does not necessarily imply 

 the verity of the myogenic theory of conduction. But this functional 

 difference between Limulus and vertebrate hearts may in fact inhere 

 fundamentally in the absence of an analogue of the vertebrate atrio- 

 ventricular conducting bundle in Limulus. 



The gross structural condition of the Limulus heart is very signifi- 

 cant in this connection. The Limulus heart consists essentially of a 

 metameric series of 9 syncytial muscular rings, for the physiologic 

 coordination of which the very scattered peripheral and central longi- 

 tudinal fibers seem quite inadequate. In the absence of a direct 

 muscular coordinating mechanism the nervous impulse to heart-beat 

 must be conducted by the longitudinal nerve-cord. 



Patten (1912) only states that the striped muscle of Limulus arises 

 very early from the somites. The histogenetic process is not described, 

 but a detailed description is given of the origin and history of so-called 

 "fiber cells," some of which give rise to definite muscles, others to 

 11 semi-amoeboid cells resembling blood corpuscles." The original fiber- 

 cells, derivatives of the germ-wall, are said to lie in the first 5 thoracic 

 segments in an intermediate zone median to the germ-wall. 



Other problems here considered are touched upon in the following 

 recent works: (i) Baldwin's (1912) on the heart-muscle of the mouse, 

 in which he describes "muscle cells" separated from the myofibrillar 

 substance by a "cell membrane"; this interpretation was shown to be 

 untenable in (2) my paper (1914) dealing with cat and mouse tissue 

 in macerated condition, and by the findings of (3) Asai (1914) in his 

 study of striped-muscle histogenesis in the mouse; (4) Thulin's (1915) 

 work on the wing-muscles of certain insects (Coleoptera) , birds, and 



