No. 3 .] 



EPIPHYSIS IN COREGONUS ALBUS. 



505 



On the roof of the brain — in the median line, and in a plane 

 passing through the middle of the optic vesicles — is seen the 

 posterior epiphysial outgrowth E. It is a small spherical body, 

 having its lateral walls thickened so that the cavity within is 

 laterally compressed. This cavity is narrowest at the middle, 

 on account of the greatest thickness of the lateral wall of the 

 vesicle falling at the middle of its antero-posterior axis ; conse- 

 quently, in a dorsal view, the cavity has the form of a dumb- 

 bell. Just in front of this vesicle, and a little to the left of it, 

 is a second similar outgrowth E'. This anterior evagination is 



Fig. II. — Transverse section through the posterior epiphysial vesicle of Coregonus albus. 



smaller than the posterior one, and appears to be solid. It lies 

 close against the wall of the posterior vesicle, and is partly hidden 

 by it. 



Fig. II. shows the dorsal face of a transverse section through 

 the middle of the posterior epiphysial vesicle. The arrow in 

 the figure indicates the position of the median plane of the 

 brain. The epiblast (Ep) lies close against the distal end of the 

 vesicle, and is made up of a single layer of flattened epithelial 

 cells. No mesenchyme is present between this epiblast and 

 the dorsal surface of the posterior epiphysial vesicle. The 

 vesicle (E) has the outline of a top, and is connected with the 

 brain roof (B) by the apex of the top, which thus forms a very 



