Eycleshymer-Davis, Epiphysis and Paraphysis in Amia. 47 



from the yolk we have failed in a number of embryos to detect 

 its beginning. 



The epiphysis (ei) first shows as a slight evagination in 

 the roof of the primary fore-brain as shown in Fig. i. The 

 appearance of this section suggests that its beginning might 

 have been primarily the result of an invagination of the brain 

 at the point x ; the two folds later assuming the form shown in 

 the figure, 



A transverse section of this stage (Fig. 2) shows a lateral 

 extension of its distal portion. There is at this time little dif- 

 ferentiation in structure. We have noticed in a number of cases 

 that the cells forming the outer layer appear somewhat larger 

 than the others. Aside from this we have not been able to find 

 any histological characters which would distinguish it from the 

 adjoining brain wall. Neither have we found in the earlier 

 stages any peculiarities by which we could predict its point of 

 origin. 



Embryo §-6 days. 



In this stage the tail of the embryo is just becoming free 

 from the yolk and the outline of the heart is barely visible. 

 The epiphysis, or primary vesicle, when reconstructed, is found 

 to be a small oblong mass of cells lying in the median line. 



A sagittal section is represented in Fig. 3. It is here ob- 

 vious that the vesicle iei) has flattened dorso-ventrally and 

 elongated posteriorly. A distal and proximal portion may now, 

 in a general way, be distinguished. The longer distal portion 

 lies nearly in a horizontal plane with its ventral wall closely 

 applied to the brain roof ; this passes over into the narrow prox- 

 imal portion which connects it with the brain wall. Its lumen, 

 which was short and conical, has now extended posteriorly, wid- 

 ened laterally, and flattened dorso-ventrally. Its opening into 

 the third ventricle is well shown in the section. It is also ap- 

 parent that the dorsal wall of the vesicle already manifests im- 

 portant changes ; throughout the greater portion of its extent 

 it has undergone a decided thinning, so that it now consists of 

 but one to two layers of cells. It is important to note that this 



