42 SYDNEY EVANS JOHNSON 



fibers of the ganglion cells pass directly to the inner surface of 

 the ectodermal thickening, where they ramify amongst the 

 cells which are already becoming differentiated to form the 

 sensory column (figs. 35, 36). This primitive lateral nerve 

 lies immediately subjacent to the cells of the sensory epithe- 

 lium. The grouping of the fibers into ramuli is a secondary 

 process of which there is as yet no indication. 



Figiu-e 36 is a longitudinal section of the growing end of the 

 lateral sensory cord, showing the ectodermal pocket, the sensory 

 column and the terminal fibers of the lateral nerve. The nature 

 of the ectodermal pocket is well shown — it is a fold of the outer 

 layer of the skin curving over the thickened growing end of the 

 sensory cord. It takes no part in the formation of the canal 

 which subsequently arises by a process of involution of the sen- 

 sory epithelium and the closure of the groove thus formed. The 

 overlying epidermal fold, although it becomes much elongated, 

 breaks down and disappears before the true canal is formed. 



The epithelium of the growing end is undergomg differentia- 

 tion. Some of the cells are assuming a columnar or a spindle 

 form, while others appear to be assuming the features of hair 

 cells. Abundant mitotic figuiTS indicate rapid growth of this 

 part of the cord. The continuity of the sensory colmnn is 

 unbroken, there being as yet no differentiation of the cells into 

 the little groups or clusters that a httle later are characteristic 

 of the sensory column. 



Seventeen millimeter stage. This embryo is shghtly in ad- 

 vance of the last stage. 



On the head the supraorbital thickening has gro^vn forward to 

 a point above the middle of the eye. Its companion cord, the 

 infraorbital, is about the same length and is dn-ected forwards 

 close to the inferior margin of the eye. Innnediately subjacent 

 to the thickened cords of ectoderm, and in close association with 

 the cells of the latter, are large bundles of nerve fibers which 

 arise from the ganglia of the seventh nerve and which accompany 

 the growing cords of thickened ectoderm. 



The lateral thickening has assumed more definite form and 

 extends posteriorly to the anterior margin of the pectoral fin- 



