524 LOCV. [Vol. XI. 



vesicle has been crowded forwards into the region of the 

 thalamencephalon. This may be seen after the removal of the 

 overlying layers of mesoderm, etc. (PI. XXVIII, Fig. 44). 

 The fifth nerve is already well begun, and nerve fibres are also 

 given off from the segments numbered 9 and 10. 



Fig. 36 shows an embryo slightly older than that in Fig. 35. 

 The line of neuromeres have been forced further apart by the 

 lateral growth of the dorsal wall of the hind-brain. The ear- 

 vesicle is no longer circular in outline, but is fast becoming a 

 closed pouch. The eye shows the beginning of the lens and 

 the choroid fissure. The Anlagen of the fifth, seventh, eighth, 

 ninth, and tenth nerves are distinctly visible from surface 

 observation. The branchial arches are all clearly outlined and 

 the first two gill-clefts have broken through. The specimen 

 shows about forty-five mesodermic somites. A line of surface 

 elevations over the hinder branchiae mark the beginning of the 

 lateral line. 



In Fig. 37 the neural segments are undergoing some changes 

 in outline that are likely to lead to confusion in identifying 

 them in later stages. If, for example, we look along the lower 

 margin of the segmented border, we shall see that the eleva- 

 tions and constrictions are substantially as they have been in 

 all the previous stages, but those along the upper margin no 

 longer correspond with them. In all the preceding figures the 

 boundaries of the segments correspond on both upper and 

 lower margins. In Fig. 37, however, the upper margin shows 

 elevations just above the constrictions on the lower margin. 

 These new-formed elevations become very quickly prominent, 

 while the segments along the lower margin lose their individ- 

 uality, and the segmented area becomes more and more an 

 irregular sinuous band with crests upon its upper margin. 

 The entire line of segments finally becomes indistinguishable, 

 but if they be studied in stages immediately following that 

 represented in Fig. 37 it will be the crests along the upper 

 margin that first catch the eye. If the observations are made 

 from above, these crests are seen to be transverse folds on 

 each side of the medulla, and when counted will, of course, 

 be one less than the original segments. It is only by viewing 



