MORPHOLOGY OF CYCLOSTOMATA. 349 



and cut off a long chamber from the body-cavity along the 

 openings of the nephrostomes (figs. 77-85, pp. l). This longitudi- 

 nal chamber communicates anteriorly as well as posteriorly with 

 the body-cavity below, which is represented, in those parts, ])y the 

 boundary of the parietal and visceral layers of the lateral plate. 

 This is the uppermost partition. The second partition is weaker 

 in development and is detected a little more ventrally, projecting 

 likewise from the parietal and the visceral layers of the lateral 

 plate. It is most obvious in the region beneath the third and 

 fourth nephrostomes (81-84, pp.2). We find the third partition 

 still more ventrally, which is weakest in development ; its extent 

 is almost the same as the second (figs. 81-84, pp.3). 



These partitions disappear after a short existence ; in a little 

 older embryo, none of them is detected, as will further be seen. 

 This is probably the same structure as the *^ pcritmieale Scheide- 

 wände " or *' Pentonealbrüche " described by Goette in Petromyzon 

 fluviatUis ('90). As to the meaning of the structure I have 

 nothing to sa}^^^ 



It is important here to illustrate the topographical position 

 of the pronephros and the relation of it to other parts ; for these 

 become definite for the first time in the present stage. For this 

 purpose, a series of sagittal sections is instructive (figs. 112-114)."' 

 A few anterior myotomes {/nt.lI-V) are seen in fig. 112, which 

 represents the section nearest the median line. In the posterior 

 part of these myotomes, four cell-layers are distinguishable ; the 

 outmost layer (ep.) is the epiblast ; the cell cord (cd.) inside the 

 epiblast is the longitudinal section of the collecting duct, and 



l)See the liistorical review under Peironyzon. 



2)Tlie embryo, from which these figures are drawn, is a little younger than tliat just 

 spoken of. 



