ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
573 
same cause does not exist on tlie ventral or lateral surfaces there cannot 
be the same result ; hence the difference between the dorsal and ventral 
walls of the intestine. The multiplication of the elements of the dorsal 
wall of the intestine causes the formation of a layer which grows in and 
leaves below it a space — the intestine — while it becomes, dorsally, 
attached to the two other layers. The curvature of the dorsal wall of 
the intestine causes the curvature of the blastopore which becomes at 
first semilunar and then circular. The blastopore persists and forms 
the permanent anus. 
The author next deals with the origin and development of the 
peripheral nervous system. He confirms the results of Beard as to 
the epiblastic derivation of the dorsal roots of the cranial nerves. 
He describes the primitive stage of the cranial ganglia as an unseg- 
mented epiblastic band which, later on, extends into the trunk to 
form the lateral nerve and line. While this cord is being differentiated 
posteriorly it becomes segmented anteriorly to give rise to the different 
ganglia, and this segmentation follows that of other parts of the head. 
The central nervous system, which is at first unsegmented, is directly 
metameric in the brain and spinal cord ; this metamerism, which is 
easily recognizable in young examples, is called “neurotomy.” The 
dorsal spinal and the cranial roots of the nerves arise behind the 
neurotome of their segment, and their relations with it are secondary. 
The ciliary and the auditory nerves have each a postbranchial branch, 
which passes behind the branchial cleft of their segments. The facial 
nerve not only has a suprabranchial branch which is double at its 
extremity, but the ganglion itself is double and has two postbranchial 
branches, one of which passes behind the hyo-mandibular cleft and the 
other behind the hyoidean. The author believes that he has removed 
any difficulty with regard to the identity of the segments of the trunk 
and of the head, and he believes that he has established the complete 
homodynamy, at least in its fundamental parts, of the peripheral nervous 
system of all the metameres of the body. 
In his third essay M. Houssay treats of the metamerism of the head, 
and discusses the principles on which the determination of the metameres 
must be based. The segments appear at different points and at different 
times and obey no simple law. There is an absolute agreement in the 
way in which the central nervous system (neurotomy), the peripheral 
nervous system (neuromery), the branchial intestine (branchiomery), 
and the mesoderm (mesodermery) become divided. At the same time it 
is to be noted that parts which typically ought to exist, retrograde or 
are even not produced at all, and thence arise errors in the theories as 
to the segments of the head. 
In addition to the cephalic segments which are generally admitted — 
the nasal, the mandibular, the hyoidean, and the branchial, the author 
brings forward evidence in favour of the oculo-hypophysial, of which he 
finds the branchial cleft, the postbranchial nerve, and the mesodermal 
segment ; of the buccal ; of the hyo-mandibular, of which he fixes the 
branchial cleft, the ganglion, and the post-brancliial branch ; and of the 
auditory. 
Further arguments are required to justify us in regarding the 
oculomotor, the trochlear, and the abducens nerves as ventral roots. 
