The Microscopic Structure of Striped Muscle of Limulus. 281 



lemma is lacking, its place being taken by a connective-tissue envelope. 

 But conditions are almost exactly like those described for the skeletal 

 muscle. Successful counter staining with picric acid-fuchsin reveals 

 internal to the red-stained endomysium an unstained, or only slightly 

 stained, delicate layer with which the telophragmata are directly con- 

 tinuous. It is very intimately connected with the enveloping connec- 

 tive tissue (fig. 10), but occasionally it may be seen separated therefrom, 

 as in figures 8, 11, and 13. When the staining is prolonged to the 

 point where no distinction can be made on the basis of a difference in 

 staining reaction between the sarcolemma and the endomysium, the 

 telephragmata also, and even the peripheral sarcoplasm, show the 

 same character and depth of stain. If we should decide on the basis 

 of such a picture that the sarcolemma is in fact part of the connective 

 tissue, we would be compelled to draw the same conclusion for the 

 telophragmata and the peripheral sarcoplasm. Moreover, contrary to 

 Meek's observation, the difference, both from the standpoint of size 

 and structure, between the muscle nuclei and the connective-tissue 

 nuclei, is striking, as can be seen from figure 13. The cytoplasm of 

 the blood-cells and of the connective-tissue cells contains iipoid spher- 

 ules similar to those of the perinuclear sarcoplasm. 



THE NUCLEUS. 



The nuclei are scattered apparently at random throughout the 

 muscle trabeculse. To their walls are attached the telophragmata, 

 as in skeletal muscle. Hence, there can be no true cardiac-muscle 

 cells. The cardiac muscle is an intricate syncytium, even to the finest 

 visible fibrillar elements of the myofibrillae. The most striking differ- 

 ence, and the only other essential difference, barring the gross syncytial 

 character, is the generally much greater length of the nuclei; as many 

 as 8 telophragmata ending on the periphery is a common condition 

 (fig. 12) ; moreover, it is common to find 4 or more nuclei closely apposed 

 in the same sarcoplasmic area. All the evidence goes to indicate exten- 

 sive direct nuclear proliferation; all stages in amitosis can be readily 

 demonstrated. Only rarely do 2 closely apposed nuclei in the same 

 sarcoplasmic area appear in the skeletal muscle. This suggests, how- 

 ever, that the method of the nuclear multiplication is the same in the 

 cardiac and skeletal type of muscle in the later growth stages that is, 

 by amitosis. Not a single karyokinetic figure was seen in the muscle- 

 tissue of either type. 



INTERCALATED DISKS. 



Meek (1909) reported the absence of intercalated disks in the heart- 

 muscle of Limulus. In this conclusion I confirmed him in an earlier 

 study (1912). After reinvestigating the subject with a different stain- 

 ing technic (iron-hematoxylin and picric acid-fuchsin), I am able to 

 report a very few disks of the very simplest or comb type (fig. 15). It 



