224 HENRY C. TRACY 



matrix. A peculiarity of the tube in the shad (fig. 2) is the 

 indefiniteness of the perichondrium; the transition from fibrous 

 tissue to cartilage matrix is gradual and matrix formation has 

 proceeded into the surrounding connective tissue in a very 

 irregular manner. There is therefore an intimate connection 

 dorsally and at the sides of the tube with the muscular aponeuro- 

 sis on which it lies. These conditions produce the roughness 

 which is characteristic of the surface of the tube as seen in dis- 

 section. It is attached to the aponeurosis by strong threads of 

 connective tissue. The dimensions of this tube in one adult 

 specimen of Alosa, measured from the section by micrometer 

 methods, is as follows: outside diameter, 0.35 mm. in one dimen- 

 sion, 0.6 mm. in the other; least inside diameter, 0.12 mm.; 

 greatest inside diameter, 0.19 mm.; diameter of the membranous 

 diverticulum, 0.05 mm. In Stolephorus, outside diameter, 0.14 

 mm.; inside diameter, 0.075 mm.; diameter of the diverticulum, 

 0.03 mm. 



THE PERIMENINGEAL TISSUE AND THE PERILABYRINTHINE 



SPACES 



The perimeningeal tissue over the top and sides of the brain 

 is for the most part undifferentiated and consists of a layer of 

 connective tissue, thick and fatty in most places, but mem- 

 branous and fascia-like over the lateral cartilage plate (Pomo- 

 lobus pseudoharengus). This fatty tissue also extends through 

 the temporal foramen and encloses the large bay of the lateral- 

 line canal which lies therein. The perimeningeal tissue extends 

 around the semicircular canals and between the outer side of 

 the sacculus and the corresponding bony wall of the auditory 

 recess; it fills the lateral recess and surrounds the expanded 

 part of the lateral-line canal in the recess. In these latter 

 situations, however, it apparently does not contain fat, but is 

 of a very loose, reticular-like structure. 



Immediately around the membranous labyrinth and under the 

 brain, a considerable part of the perimeningeal layer is dif- 

 ferentiated into a tissue which is similar to that forming the 

 tunica propria of the epithelium of the labyrinth in other ani- 



