MORPHOLOGY OF CYCLOSTOMAÏA, 335 



opposite side (fig. 53, pt.2). In short, in this series of sections, 

 tlie tubnles on the right side, are all more advanced than those 

 on the opposite side. Tlie sixth is very primitive in development ; 

 fig. 57 represents the section, on the left side, through the anterior 

 part of the ninth somite and, on the right, the posterior part of 

 it. The left tubule is sliced at its anterior wall, but the right 

 tubule is cut through in its mid-plane. Tt is composed of two 

 layers of columnar cells, but no cavity has yet appeared in 

 the interior. 



From the tenth somite backwards, the Anlagen are cut off 

 from both the myotomes and the lateral plate, and constitute the 

 segmental duct or the posterior continuation of the collecting 

 duct, which is distinctly traceable for 7-8 somites. Not infre- 

 quently, however, a somite is met with, in wdiich the segmental 

 duct is not yet cut off from the lateral plate at the ti.iie wdien 

 the separation is finished in a majority of somites, as seen in 

 fig. 58 which represents a section through the twelfth somite. 

 The left half of the figure show^s the duct entirely cut off from 

 the lateral plate, while the right exhibits the state not yet 

 separated. The same structure is made out in two contiguous 

 sections, so that one might mistake it for a pronephric tubule. 

 This point will be described further on. 



The relation of the pronephric tubule and the peritoneal 

 cavity is not so simple as in the last specimen ; besides the 

 pronephic tubule, there is seen another structure which projects 

 out of the inner angle of the peritoneal cavity (figs. 52, 53, 6by 

 and bQ, c.p.). This projection is originally a fold of the peritoneal 

 \vall and gives rise, as subsequent history shows, to the radix of the 

 mesentery, whence the gonads and the mesonephric tubules are 

 derived. It will here be called briefly the " coehmic projection.^^ 



