ORIGIN OF JAW APPARATUS 401 
skeleton of considerable extent; but as related to vertebrates 
above the Myxinoids it represents only the early stages in the 
ontogeny of their head skeleton. This is as one would expect. 
Therefore the development of the craniofacial apparatus in 
Bdellostoma is not an abnormal feature of vertebrate morphology 
either from the viewpoint of: palingenesis or ontogenesis. It 
is in harmony with the main current of cause and effect which 
we call evolution of structure. 
This is clearly shown by a comparison of the musculature, 
nerve and vascular supply and the distribution of the connective- 
tissue sheets and tendons, indicating the directions of the lines 
of stress and the location of future structural changes, for where 
stresses are developed new structural arrangements are evolved 
and hard parts come into being to take up and transmit the 
major stresses. 
As the embryo of Bdellostoma is flattened between egg-shell 
and food yolk, there is no possibility of development in full form 
as occurs in the free embryo of Ammocoetes and larvae of Pet- 
romyzon. ‘The jaw mechanism is flattened and vertically com- 
pressed under the overgrowing brain, but the early larval stages 
show that it grows cephalad and soon projects beyond the anterior 
end of the brain. As the food yolk is absorbed and a place made 
for the growing head, the parts expand dorsovertrally as well, 
the snout region projecting into the space left free by the absorp- 
tion of the yolk. The jaw mechanism passes through the Am- 
phioxus and Ammocoetes stages, which are functional structures 
in these animals in early embryonic life, by a series of rapid 
transformations so that the structural characters of the ancestral 
stages represented by Amphioxus and Ammocoetes, while marked 
out, do not progress beyond embryonic tissues and by the time 
definitive cartilage and muscle tissue appear the characteristic 
structure of the Myxinoid jaw mechanism has been established. 
There is apparently no remnant of a latent period, such as occurs 
in the larval Petromyzon, even if such a larval state ever formed 
a part of the life-history of the ancestral Myxinoids. 
The jaw bars occupy the same relative position about the 
mouth of the Bdellostoma embryo as in Amphioxus and Ammo- 
coetes. From the basal part the jaw bars pass out and forward, 
