MORPHOLOGICAL STUDIES. 
165 
The stages of development which Balfour described as the 
earliest are, however, by no means such, for I can demonstrate 
the first traces of ganglia some time before the neural canal 
closes. Neither Balfour norOnodi, 1 nor any other observer, has 
seen the stages which I figure in PI. XVI, and in figs. 37 — 42 
of PI. XVIII. 
Figs. 1 — 4 of PI. XVI are taken from various parts of one 
embryo of Torpedo ocellata. Figs. 1 — 3 are the only ones 
which at the moment concern us, for they are all three from 
the trunk, and hence from the region of the spinal ganglia. 
In Prof. His’s recent paper (No. 34, p. 445) the author 
remarks : “ Der Zeitfolge nach entwickeln sich die periphe- 
rischen Nerven spat. Am Rumpf treten sei spater auf als die 
Urwirbel, am Kopf fallt die Zeit ihrer Bildung zum nahe 
an diejenige des Visceralbogen, aber da geht die Gliederung 
des End-gebietes dem Vordringen der Stamme voraus.” 
I am not quite sure that Professor His means these remarks 
also to apply to the ganglia. But however that may be, I will 
at once assert that the “ Anlagen” 2 of the spinal ganglia are 
formed very much earlier than has hitherto been supposed, 
and, indeed, that the first traces of them appear when only two 
or three of the mesoblastic somites 3 have been entirely seg- 
mented off from the main raesoblast (figs. 2 and 3). Generally 
speaking, the first differentiation of the spinal ganglia may be 
said to occur at about the time of separation of the notochord 
from the hypoblast. In earlier stages than this fig. 1 (here the 
1 Onodi’s researches, so far as they relate to the posterior root-ganglia of 
Elasmobranchs after exclusion of the sympathetic, contain no new results 
And their author was entirely in the dark as to the relations of the ganglia 
to the lateral sense organs. Though he must have seen the skin fusions he 
entirely ignores them. 
s I use the word Anlage or Anlagen (plural) throughout this paper instead 
of our only term rudiment, which has a double meaning. 
3 In agreement with van Wijhe, Wiedersheim, and others, I use here the 
word somites, or body-somite, instead of the older and incorrect term proto- 
vertebrae. In the same way I shall call the “ head-cavities,” with van Wijhe, 
the head-somites. With Dr. Eisig 1 use the terms haemal and neural instead 
of ventral and dorsal. 
VOL. XXIX, PAHT 2. NEW SER. 
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