Eycleshymer-Davis, Epiphysis and Paraphysis in Amia. 5 5 



39, 40. The positions of the sections are indicated by their 

 numerals in Fig. 45. The sections show that the primary ves- 

 icle is inclined toward the right. The structure as a whole has 

 increased in size, but this is due largely to the growth of the 

 stalk. The walls of the vesicle retain the same relative thick- 

 ness as in the preceding stage. Its lumen is better defined and 

 is apparently filled with a coagulum. The layer of cells lining 

 the cavity of the stalk and those forming the ventral wall of 

 the vesicle have elongated and become pear-shaped in general 

 outline, with their smaller ends pointing inward. 



The secondary vesicle has not changed its relative position. 

 It has narrowed somewhat and appears longer dorso-ventrally. 

 For the first time a well defined lumen is present. We have 

 not been able to find any trace of a communication with the 

 lumen of the primary or to find an opening into the thalamo- 

 coele. The cells lining this cavity are likewise elongated, with 

 their narrower ends directed inward. At one point, as shown 

 in Fig. 39, the superior commissure is in direct contact with 

 this vesicle, and several nerve fibers enter it, but can be traced 

 only a short distance. 



It is of interest to note that the cartilage {c) just above the 

 primary vesicle is much thinner than in other portions, being 

 hollowed out, so to speak, for the reception of its distal portion. 



Paraphysis. The growth of the paraphysis has kept pace 

 with other parts of the brain. It has broadened distally and 

 the two diverticula noticed in the preceding stage are almost 

 equal in size and project on either side, so that the whole 

 structure becomes bilaterally symmetrical. In some sections of 

 this stage there are indications of the formation of secondary 

 diverticula. 



Larva jo-j^ days, 22-2jmm. 

 Fig. 41 represents a diagram of the roof of the third ven- 

 tricle. The primary vesicle is somewhat larger than in the 

 preceding stage, but in general form it is essentially the same. 

 It stands out less prominently owing to the fact that the cere- 

 bral hemispheres have grown up around it. It shows a contin- 



