Eycleshymer-Davis, Epiphysis and Paraphysis in Amia. 5 1 



secondary vesicle shows no histological changes beyond the con- 

 dition represented in Fig. 10. 



The inner layer of the epithelium [ep) has now extended 

 completely over the vesicles. There is a noticeable increase in 

 the degree of pigmentation, but as yet it has not appeared in 

 the area over the primary vesicle. 



Transverse sections of a corresponding stage are shown in 

 Figs. 16, 18, 19. These are taken along the lines indicated by 

 their respective numerals in Fig. 13. They show the relative 

 size and position of the two vesicles. Fig. 17, taken along the 

 line 17-17 of Fig. 15, represents an oblique section of the pri- 

 mary vesicle and possibly the continuation of its cavity within 

 the secondary. Of this we are in doubt since we have not ob- 

 served the lumen of the secondary vesicle until a much later 

 stage. Just beneath the posterior margin of the primary vesi- 

 cle there is a thickening of the brain wall. The upper layer of 

 cells constituting this portion are elongated dorso-ventrally ; their 

 nuclei occupy the lower portions of the cells while their upper 

 ends become granular. This tract is the beginning of the pos- 

 terior commissure {p. c). 



While the usual conditions are as above described, we have 

 series which show marked variations in the position of the sec- 

 ondary vesicle, in that it lies on the opposite side of the pri- 

 mary. Furthermore we have noticed that in some cases the sec- 

 ondary vesicle is scarcely visible and seems no further developed 

 than in the 6 mm. stage. 



Paraphysis. The so-called "paraphysis" is described by 

 Hill as first appearing in a 13 mm. larva. We have found the 

 structure well defined in the 8 mm. stage. It arises in Amia, 

 as in Amblystoma, as an evagination of the posterior portion of 

 the roof of the prosencephalon. Its basis is foreshadowed by 

 a considerable thickening of the brain wall. This thickened 

 tract consists of a single layer of columnar epithelium, the cells 

 of which, aside from their being elongated, resemble those in 

 the adjacent brain wall. 



