1891.] Recent Studies of the Vertebrate Head. IOI 
muscles of the third head-cavity, which is also composed of 
several myotomes. Both nerves spring from the anterior 
columns, and are homodynamous with motor spinal nerves. The 
IV. nerve emerges on the dorsal border of the brain, but 
whether it is homodynamous with the cranial ganglionic motor 
fibres, or with the motor spinal nerves, is uncertain. The gan- 
glionic motor fibres, viz., those of the V., VII—VIIL., IX., and 
X., arise from the lateral columns. These fibres greatly converge 
in passing to the ganglion-anlagen of the respective nerves, and 
it may be assumed that at one time the fibres arose as separate 
nerves, each belonging to its myotome. Marshall believes the 
olfactory nerve to be an outgrowth from the anterior portion of 
the neural ridge. Beard advances the same view. His found 
that in human embryos the olfactory ganglionic cells and nerve- 
fibres originated from the epithelium of the nasal vescicle. 
Dohrn confirms the same in Selachians. Rudimentary ganglia 
are found in the anterior part of the V. Anlage, in the anterior 
part of the VII. Anlage, and in the anterior part of the IX.and X. 
We see indications of a centralizing process which has resulted 
in the reduction in number of the primitive ganglia. Displace- 
ment and suppression has taken place in the visceral mesoderm 
of the head. The premandibular, mandibular, and hyoid head- 
cavities are to be considered as multiples of original head-cavi- 
ties, in which serial origin the lateral plates share. The fact that 
the embryonic vascular system is similar throughout would 
indicate that it originated at a time when the body was not yet 
differentiated into metameres. The difference in direction of the 
blood-currents in the aorta and the carotids can be explained by 
the hypothesis that the current in the latter has been reversed by 
the suppression of preoral arterial arches. By this hypothesis it 
may be assumed that at one time there was no separation be- 
tween aorta and carotids. In consequence, the existing mouth is 
derived from the coalescence of one or more pairs of gill-clefts, 
and there must have been a time when the present mouth did not 
exist. Thyroid and hypophysis must have.had a bilateral struc- 
ture, so that a median passage could be left for the conus arte- 
tiosus. The aorta shares in two segmentations: one that of the 
