MORPHOLOGICAL STUDIES. 
183 
of both cranial and spinal ganglia can nearly always be ob- 
tained. 
The mode of preparation, which in Sharks is not of such 
importance, is here a very weighty factor. My embryos were 
all prepared by immersion for from half an hour to two hours in 
Flemming’s chromic-osmic-acetic acid mixture, and afterwards 
stained with borax carmine or picro-borax carmine. Osmic 
acid must be used here, and used very carefully, or otherwise 
no guarantee can be given that all the appearances depicted by 
me in Plates XIX, XX, XXI will be visible. Thin sections are 
of course also of importance, and I must express the opinion 
that the results obtained by Professor His ten years ago (No. 29) 
are vitiated by improper treatment of the embryos and by the 
thickness — at that time unavoidable — of the sections. My 
sections are mostly mm - thick. As was the case in the 
account of Elasmobranchs, I shall begin this part of the paper 
also with the 
II, a. Development of the Spinal Ganglia in the 
Chick. 
The appearances about to be described may be even seen some- 
times in embryos in which no body-somites are as yet formed, 
and, speaking generally, an embryo with about six body-somites 
will show in different regions the appearances presented in 
seven sections (figs. 70 — 76) taken from the spinal region of 
such an embryo with six mesoblastic somites. It will be noticed 
that the medullary canal is everywhere open, and, in fact, here, 
a9 in Sharks, the first traces of the cranial and spinal ganglia 
are formed long before the closure of the neural plate. The 
first section is in the region of the primitive streak — and here 
no trace of ganglionic Anlagen is to be seen (fig. 70). The 
next section (fig. 71) is taken much farther forwards, and on 
the left side of the section, at any rate, the commencement of 
the ganglionic differentiation (fig. 72, g. a.) can be seen. The 
third section (fig 72) passes through the middle of a meso- 
