"17 
weight must be attached to this testimony. Personally I think that 
the collective evidence which I now propose to state (under five 
headings) amounts to proof. 
(A) Though the facts of ontogeny rarely afford conclusive evidence 
as to the past history of any given structure, owing to the modifica- 
tions in the palingenetic series of changes which may be induced by 
the conditions of development in the organism, yet the very fact that 
ontogeny does not in any way countenance a given theory is of value: 
moreover the facts of. ontogeny may be useful as affording confirma- 
tion of conclusions arrived at from other evidence and it is solely 
for -this reason that I will describe briefly the development of the 
piston musculature and piston. The piston musculature is developed 
on the mid-ventral side of the 
gut just in front of the heart and Ur, 
shows no dorsal or lateral exten- ¥& — 
sions. As the pericardial region, 
with the gills, shifts backwards 
owing to the elongation of the neck 
region, the muscles also extend 
backwards as the result of the modi- 
fication of embryonic tissue situa- Cactilage 
ted in the mid-ventral line (Busor, Fig. I. (after Stockard) 
(5), Dean, (7) and in the adult Fig. 1 (after Srockarp) shows in 
streteh from the region of the transverse section the position of the 
: : developing piston cartilage in the floor 
moutheavity to the region of the of the buccal cavity of Bdellostoma. 
anterior gill pouch (Myxinoids) or 
to the pericardium (Petromyzontes). This course of development appears 
to me to be quite inconsistent with the view that the piston musculature 
was primitively attached to the skull (which must have degenerated 
considerably if it ever supported masticatory muscles!) and worked a 
mandible. It is indeed inconceivable that muscles underlying the gut 
and extending right back, in Petromyzontes, to the pericardium, should 
ever have been associated with a jaw. Also we may well wonder 
how the animal fared during the transitional period in which the man- 
dible was becoming converted into a dentigerous piston and its muscles, 
after relinquishing their connection with the skull and growing back- 
wards, acquired new attachments posteriorly. As regards the piston 
cartilage, it originates, according to STocKArD, as a thin plate situated 
in the floor of the buccal cavity, the lateral edges curving upwards 
