THE OLFACTORY NERVE OF LEPIDOSTEUS 119 



that mitoses are numerous in this region of the placode, contiguous 

 to the origin of the ventral olfactory ramus. It is worth while to 

 note that this is the next succeeding stage in the series to the 10 

 nmi. embryo described by Landacre ('12, p. 6) as occupjing an 

 intermediate stage in the formation of the epibranchial placodes. 

 We have then, a parallel history in the olfactory placode, for there 

 is no diflriculty from this stage on to the adult, in recognizing the 

 cells of the ganglion of the nervus terminalis. 



The next stage, no. 28 of the series, six hours later than the 

 previously-described stage, and 1G() hours after fertilization, is 

 critical in reference to the formation of the ganglion. Four adja- 

 cent sections of the same embryo have been drawn and numl)ered 

 consecutively from the most anterior to the most posterior in 

 figures 8, 9, 10 and 11. Figure S shows a greater number of cells 

 in the course of the ventral ramus of the olfactory nerve than in 

 the dorsal ramus, just as noted in earlier stages. A blood vessel 

 enters the brain opposite the ganglion and is a conspicuous land- 

 mark in later stages. The next posterior section (fig. 9) has a 

 still larger number of ganglion' cells in the ventral ramus, which is 

 the only one drawn. The line of demarcation between the placode 

 and the cells of the ganglion is not so marked as in the previous 

 section. This line is less prominent in the next posterior sections 

 (figs. 10, 11). In these posterior sections the number of ganglion 

 cells is more numerous and they produce a slight swelling on the 

 ventral ramus. They are slightly larger than a majority of the 

 cells in the placode and rival those of the neural tube in size. But 

 in the ventral part of the placode (fig. 11) are a few cells that are 

 the same size as those in the olfactory nerve. They are on the 

 border line or near it and appear to be migrating from the placode 

 into the ventral olfactory ramus to become constituents of the gan- 

 glion of the nervus terminalis. This is the best and only evidence 

 as .to the origin of the ganglion of the nervus terminalis in Lepi- 

 dosteus. At about this age the cells of the ganglion begin to acquire 

 the characteristic vesicular nuclei of neuroblasts and chromatin 

 granules are deposited in the cytoplasm. 



A study of the following five stages of the series, covering a 

 period of 48 hours, reveals the same essential relations to the 



