A COMPARATIVE STUDY OF STRIATED MUSCLE. 37 
refracting and doubly refracting material, the latter staining 
darkly with hematoxylin or carmine, the former not at all. 
Of these segments there are in the species under consideration 
from fifteen to twenty of each kind in the fibre; the doubly 
refracting zone is on an average ‘025 mm. in breadth and the 
singly refracting ‘(0083 mm. Running transversely through 
the fibre at right angles to its long axis is a narrow layer in 
the middle of each singly refracting segment which is doubly 
refracting ; this is Krause’s membrane, the transverse network 
of Retzius. These transverse networks have a quite measur- 
able thickness, being on an average 0:0027 mm. in thickness. 
The distance between successive networks, averaging 0:0333 
mm., is very great as compared with the corresponding mea- 
surement in Arthropoda and Vertebrata, where it does not 
exceed 0:005 mm. in any cases that I have examined;! and in 
fact the striation in Syllis corruscans, though it cannot be 
seen with the naked eye, can be made out with a pocket-lens 
magnifying four or five diameters. 
These transverse networks are visible as rows of granules in 
the fresh fibre ; in fibres treated with bichromate of potash and 
examined in glycerine they become more distinct, being marked 
off sharply from the surrounding substance by their refrangi- 
bility and by their yellowish colour; it becomes evident in 
such a preparation that we have to do with transverse parti- 
tions running through the whole thickness of the muscle sub- 
stance. 
In bichromate of potash specimens strongly stained with 
hematoxylin by Heidenhain’s method (figs. 1 and 2) the 
transverse networks become much more darkly stained than 
other parts of the muscle, and thus become very prominent. 
In such preparations, either teased and crushed by blows on 
the cover-glass or cut into thin sections, it becomes evident 
that in the transverse networks we have to do with partitions 
which lie partly in the interfibrillar spaces, partly in the 
1 Engelmann, however (“ Contractilitat und Doppelbrechung,” ‘ Pfliiger’s 
Archiv,’ xi), states that the thickness of the isotropous layer in the 
Arthropoda is sometimes 0°005 mm., and even more, 
