50 Journal of Comparative Neurology, 



served, in the median line. It is slightly flattened antero-posteri- 

 orly, so that its long axis coincides with a transverse section of 

 the head. 



A sagittal section (Fig. lo) taken along the line lo-io of 

 Fig. 9 shows a considerable increase in the size of the primary 

 vesicle. Its ventral wall is much thicker and its lumen nar- 

 rower than in the earlier stages. The secondary vesicle appears 

 as a mass of cells which, as yet, posesses no lumen beyond that 

 shown in this section. Its cells appear in all essentials like 

 those forming the primary vesicle. This section confirms the 

 surface observations as regards the distribution of pigment (/>). 

 It is noteworthy that the pigment is discontinuous over the pri- 

 mary vesicle while it extends entirely over the secondary. 



Fig. 1 1 represents a transverse section along the line i i-i i 

 of Fig. 10. In this, as in the sagittal section, the lumen appears 

 to be very narrow and is bounded above by a single row of 

 cells. Fig. 12, taken along the line 12-12, passes through the 

 secondary vesicle, and further illustrates its form and structure. 



Larva 10 days, 8 mm. 



The continued formation of pigment in the head region, 

 excepting the area to which we have already referred, serves to 

 more sharply define the position of the underlying vesicles. 



The reconstruction ( Fig. 1 3) shows that the primary vesi- 

 cle {ei) has elongated antero-posteriorly. The secondary vesi- 

 cle {e2) has maintained its general form but has shifted its posi- 

 tion, as shown in the figure, so that its long axis now forms an 

 angle of io°-i5° with the long axis of the body. 



A sagittal section (Fig. 14) in the plane indicated by the 

 line 14-14 shows, in a general way, the relation of the vesicles 

 to the brain roof Fig. 15 is a more highly magnified view of 

 the vesicles. The ventral wall of the primary vesicle shows a 

 continued increase in thickness, while the dorsal still, remains 

 unchanged. The continued expansion of the brain roof has 

 caused an approximation of the walls of the stalk and a conse- 

 quent obliteration of the cavity of the primary vesicle. The 



