402 . HOWARD AYERS 
tapering as they go, to meet in front of the mouth behind and 
below the nasal aperture. The tentacles bud out from the bars. 
They are four in number in nearly all stages after they appear 
and it is uncertain if more are formed, although there are indica- 
tions of two or three similar buds near the nasal end of the bars. 
They do not develop far, even if-they are tentacular buds, and 
soon disappear. The basal section of the jaw bar is broad and 
the two attach to the hyoidean base plate in the earliest stages 
I have dissected. 
We thus find, even in early larvae, that the jaw apparatus 
has the characteristics of the adult. It is desirable to carry 
the study of the jaw structures back to their first appearance as 
differentiated organs. 
A few words about the brain. 
That seetion of the spinal cord of Amphioxus from which the 
five nerves of the jaw apparatus issue forms in Bdellostoma the 
anterior part of the medulla. As a result of the growth of the 
brain vesicle in the descendants of Amphioxus this medullary 
territory has been formed out of the symmetrical spinal cord by 
the pressure of the posterior part of the brain wall in such fashion 
that the lateral walls of the spinal cord have been spread apart 
with the extended and thinned dorsal wall stretched between their 
edges. The ventral wall of the cord, being thick and resistant, 
is not disrupted, remaining intact. The brain crowding back into 
the V-shaped space thus formed has slid backward over the 
ventral portion of the cord, leaving it unaffected. This process 
can be observed in the ontogeny of many living forms. An 
excellent series of figures illustrating the effects of the mechani- 
cal stresses produced by the increase in volume of the forebrain 
vesicle is given by Kerr for Lepidosiren. Of course, in the higher 
vertebrates the process cannot be seen, or up to date has ‘not 
been seen in full detail, as the nerves leaving the medullary 
segments are already much crowded when recognizable. 
Nevertheless, the medullary region is at first a cylinder and 
is later split open on top to form the medullary V. In all cases 
the trigeminal complex leaves the prongs of the V, which usually 
form terminal lobes. At all times during this growth the brain 
and cord are under tension, due to being inclosed in their en- 
