66 CHARLES BROOKOVER 



elements are outside it and the nuclei within it among the nerve 

 fibers belong to the sheath cells. This membrane, however, can- 

 not always be made out. When this is the case, it is difficult to 

 differentiate the mesenchyme cells from the cells among the fibers 

 of the nerve. Their nuclei are about the same in size, but in 

 favorable preparations stained with iron hematoxylin the nuclei 

 of the mesodermal cells appear darker with a greater number 

 of chrolmatin granules in them. The sheath cells within the 

 nerve have a rather clear nucleus with one or two nucleoli. 



Unfortunately an accident seems to have befallen the male fish 

 guarding the nest from which the stages above described were 

 taken and a turtle was found in possession of the nest about 

 thirty hours after hatching. This necessitated taking eggs from 

 a different nest for the later stages. This second series began 

 during the second day after hatching and was continued for 

 more than a week. They were preserved in Zenker's fluid at 

 intervals of about four hours for the first two days. They were 

 stained as in the previous series. 



In the first stage of this series it is evident that there has been 

 a large increase in the number of cells in the course of the olfac- 

 tory nerve (fig. 6) , but the distribution is the same as in the last 

 stage of the previous series. The plane of section does not show 

 the whole course of the olfactory nerve. The cells are more 

 numerous peripherally but are entirely absent near the brain 

 wall. The brain wall has raised up into a cone pointing into the 

 olfactory nerve. This cone is an early stage of the olfactory 

 bulb and its apex is devoid of cells, although there are numerous 

 neuroblasts at its base where the mitral cells are forming. The 

 mitral cells can be distinguished by their larger size and vesicular 

 nuclei, aside from the fact that a wide fiber zone free of nuclei 

 intervenes between them and the nuclei in the olfactory nerve. 

 ( onsequently, there does not appear to be any evidence at this 

 stage that cells are migrating from the brain into the olfactory 

 nerve. 



In sections of one or two of the fishes of the earliest stage of 

 this series, there can be seen a slight aggregation of cells among 

 the peripheral olfactory fibers at their anterior side (fig. 6, a). 



