MORPHOLOGICAL STUDIES. 
175 
such traces were not differentiated in the section from which 
he figured. In his monograph of the development of Elasmo- 
branch fishes, Balfour has also given, on PI. IX, many figures 
of stages corresponding to those on my PI. XYI ; but here again 
no trace of the ganglionic Anlage, which is seen in all my 
figures, has been represented. 
In fact, of the cranial, just as of the spinal ganglia, no 
observer has hitherto seen the very first stages which I am 
about to describe, and the last observer, Onodi (No. 51), who 
has given no figures at all, has, judging from his description, 
only seen the Anlagen in much later stages, and, as we shall 
afterwards see, has not interpreted rightly or seen all that is 
to be seen in fairly decent sections. 
Returning to fig. 4, we find, on examination, the same 
appearances {g. a.) as were met with in the developing spinal 
ganglion. If we examined an earlier stage than this we should 
meet with no trace of the Anlagen of the cranial ganglia. We 
see now a central portion which represents the brain part of 
the neural plate in section. At each side of this, but inde- 
pendent of it, one notices the budding out and separation, so 
far as the lateral epiblast is concerned, of a process which is, 
as we shall see, the first trace of a cranial ganglion, or rather 
of part of one. Soon after this phase the involution of the 
neural plate begins just as in the case of the spinal cord, and 
along with the involution the ganglionic Anlagen are also 
carried upwards. I have figured these stages in figs. 6, 10 — 12, 
16, 19, 20, 39 — 43, taken from various parts of the brain, in 
order to show that this mode of development holds for portions 
of the olfactory ganglion (figs. 19, 20), mesocephalic or gan- 
glion of the ophthalmicus profundus (No. 7), trigeminus, 
facialis, auditory, glossopharyngeus, and vagus. 
The involution of the neural plate, on its completion, 
encloses the cranial ganglionic Anlage just as occurs in the 
spinal cord. A number of figures of this stage are given in 
figs. 1 8, 21 , 24, 25, 29, 45, 48, g. a. 
The Anlage is now separated from the skin, and in the head 
of Elasmobranchs no trace of a Zwischenstrang is left behind. 
