Albert C. Eycleshymer and James Meredith Wilson 157 
mentally different conceptions which are at present beyond proof or dis- 
proof, we may dismiss the question without further comment. 
Concerning the neuromeres which are so well shown in Fig. 19, we 
ean only say that at present we are unable to interpret these structures. 
They have not been found in the preceding stages and have not been 
followed in the sueceeding stages. Why they should appear at this time 
and be wanting in the stage shown in Fig. 18, is at present unexplain- 
able. It is impossible to state whether they are secondary foldings due 
to the formation of protovertebree or whether they are formed inde- 
pendently in the floor of the hind brain and are the first definite expres- 
sion of segments. If the latter be true, as many embryologists hold, 
then we should find in the hind brain of Amia indications of seven or 
eight primitive segments. 
The median pit, which first appears in Fig. 42, has been followed in 
both the earlier and later stages. After a careful study much doubt lin- 
gers in our minds as to whether or not it takes any part in the formation 
of the hypophysis. Kupffer maintains that in Petromyzon and Acipenser 
this structure forms the hypophysis. It seems to us possible that the in- 
vagination of the gut to form the median portion of the adhesive organs, 
as shown in the figure, would carry the epiblast outward in such a man- 
ner that it results in an increase in this invagination. In other words, 
the mechanical factors operating could cause just the appearance ob- 
served. We should hesitate to regard this structure in Amia as of great 
value in phylogenetic interpretation. 
The previous observations on the development of the optic vesicles in 
Amia are embodied in the following sentence by Dean, 96: ‘“ The mode 
of development of the eye and of the nasal and auditory capsules differs 
but little from that typical in the lower vertebrates generally.” Our 
studies show that the eyes first appear as solid outgrowths which shortly 
after become hollow. 
Concerning the early development of the auditory vesicle there is 
nothing beyond the sentence quoted above. According to our observa- 
tions, the ear likewise begins as a solid thickening of the deep epiblast 
over which the superficial layers pass unmodified. This thickening con- 
tinues until there is an oval mass lying on either side of the anterior por- 
tion of the medulla. When the embryo reaches the stage shown in Fig. 
18, a cavity is present. 
The olfactory organs first appear as proliferations of the deep. ecto- 
blast in the stage represented in Fig. 19. In this mass an invagination 
soon appears forming well defined pits. ; 
