llf) CHARLES BROOKOVER 



increase in the number of cells in the neural tube and in the nasal 

 ])lacode but only a slight increase in the thickness of each. The 

 embryo has increased scarcely at all in length, as indicated by a 

 table of lengths made for the series of embryos, but it has expanded 

 in the head region so that the mesenchyme forms an open network. 

 This network fills a considerable space between the neural tube 

 and the nasal placode. This space is narrowed somewhat where 

 the neural tube rises up to meet the fibers of the olfactory capsule. 

 The olfactory fibers are disposed in two rami which persist with 

 considerable distinctness from this stage forward. Blood vessels 

 can be recognized near the ventral root. A line of demarcation 

 between the placode and the ectoderm appears and the latter has 

 begun to vacuolize to form the external nasal opening. Mitoses 

 are found in the deeper portions of the olfactory placode near the 

 origin of the fila olfactoria, which occur in but two sections, but no 

 karyokinetic figures are found in the neural tube beyond the 

 germinal region close to the lumen of the canal. 



Three intermediate stages have been studied but the same 

 essential relations exist until we come to the age of 148 hours. In 

 sections of this age a number of cells are to be seen on the ventral 

 ramus of the olfactory nerve (fig. 3) at the point where it arises 

 from the placode. If an occasional cell is found on the dorsal 

 root, their numbers are never so great in any of the embryos of 

 this age examined. However, it is onh^ by reading from the older 

 stages where there is no difficulty in recognition of these cells in a 

 compact ganglion, back into the earlier stages that one would 

 notice the greater number of cells on the ventral ramus. It has 

 not been possible to discriminate between the mesodermal cells 

 lying near the ventral ramus, and the ganglion cells at an early 

 age, although a number of stains have been used upon the material 

 at hand. Yet some of these cells lying among the fibers of the 

 fila olfactoria are not mesodermal elements and are the earliest 

 recognizable cells of the ganglion of the nervus terminalis. 



Six hours later the cell aggregation at the surface of the ventral 

 ramus (figs. 4 and 5) is not appreciably larger and the two adja- 

 cent sections drawn are the only ones showing any cells to l)e 

 attributed to the ganglion of the nervus terminalis. The more 



