74 MEMOIRS OF THE NEW YORK ACADEMY OF SCIENCES 



to such a rule, however, is found in the case of Mursena ? a form having a rapidly 

 developing pelagic egg, for this fish exhibits, in the formation of its vesicle, the 

 hypothetical primitive condition more clearly than any other tcleost that has been 

 carefully described. 



I have observed both in Mursena ? and in the trout an incompleteness of the 

 lower wall of the vesicle at one period. The cells forming the floor of the vesicle 

 are embedded in the underlying periblast (figures 13 and 14, A) and are not at 

 all points in contact with one another. This fact, and the presence of free cells in 

 the periblast in this neighborhood, might lead to the suspicion that the lower wall 

 of the vesicle is formed by cells derived from the syncytium. In fact, this is a 

 conclusion adopted by REINHARDT ('98). I do not find any evidence that these free 

 cells are formed out of the periblast proper. They are present from a very early 

 period in the development of Salvelinus, and occur beneath the whole blastoderm, 

 but especially at the posterior end. It seems probable that their differentiation as 

 cells dates back to the segmentation period. If so, they are to be regarded merely 

 as detached segmentation spheres which have lost all connection with the blasto- 

 derm. They are surrounded by distinct cell-walls and generally contain normal 

 nuclei. Whether such free cells play any part in the building of Kupffer's Vesicle 

 in Salvelinus or Mursena f I cannot say definitely. In Amiurus and Noturus such free 

 cells are of very rare occurrence. I have observed them only once or twice in all the 

 Amiurus eggs examined, and do not recall having seen any in the eggs of Noturus. 

 OELLACHER ('72) was probably the first to describe this conditional though he un- 

 doubtedly made no distinction between these embedded cells and the periblast nuclei. 



Such an incompleteness in the ventral wall may be looked upon as a condition 

 transitional between the two types of vesicle described. SOBOTTA ('98) has already of- 

 fered as a transitional form the case of the rainbow trout, which exhibits a complete, 

 though extremely thin, lower wall. That such cases as I have described may have, 

 however, some other morphological significance is suggested by the occurrence of a 

 like condition in Amia. Here I find, in both transverse and longitudinal sections 

 that a gap in the cellular floor of the vesicle occurs along the median line, the floor 

 being completed by the syncytium (figure 17). 



VII. YOLK VESICLES. 



In certain fishes, there exists, in addition to Kupffer's Vesicle, a second vesicle 

 lying in the yolk beneath the floor of the former. This structure is well shown in 



