PINEAL GLAND. 
1177 
the Lampreys as the basis of his explanation of the corres- 
ponding structures in the higher vertebrates: — and even he 
could declare that “the structure of the brain in the Lam- 
prey and Pride is a perfect parallel to the earliest stages 
in the development of the brain of the higher vertebrates.” 
From the results of the history of evolution Beard" 
drew a picture of the worm-like ancestors of the ver- 
tebrates, ancestors which have all disappeared from the 
earth, so far as we know, without leaving a trace of 
their existence. Their body, he says, was anteriorly 
furnished with several, at least eleven pairs of gill-clefts, 
and above, i. e. at the dorsal termination of each gill- 
cleft, lay a sense organ in the skin, probably to warn 
the animal of the approach of any dangerous object 
towards the gill-bearing parts of the body. The nearest 
guide, sense organs of the same kind have originated 
the development of all the other higher senses. Even 
in the mammals Froriep 4 has traced the same course 
of development for three of the cranial nerves ( facialis , 
glossopliaryngeus, and vagus) as Beard found to obtain 
in Rays and other fishes for all these nerves, including 
those of smell, hearing, and taste. In order to obtain 
their basal ganglion these nerves grow from the sides 
of the rudimentary spinal canal downwards and out- 
wards to the inner surface of the embryonic skin and 
meet a thickening thereof at a spot answering to a prim- 
ordial sense organ of a gill-cleft. The ganglion is form- 
ed by the growth and multiplication of the epidermal 
cells at this point; but the rudiment of the primordial 
sense organ disappears after having thus done its duty 
C 
Fig. 347. Three sketches of the brain and medulla oblongata in a Pride ( Petromyzon in the Ammoccetes stage), magnified. After Wieders- 
heim. A, from below; B , from the right; C, from above. 
I — X , cranial nerves in order from in front; 1, first pair of spinal nerves; Cb, cerebrum; Cbl , cerebellum; Ep, epiphysis; Hyp , hypophysis; 
L. ol., lobi olfactorii; ME, mesencephalon; Med, medulla; Mob, medulla oblongata; S. v., saccus vasculosus; Thl, thalamencephalon. 
parallel to the sense organs of the gill-clefts in their 
original form is thus afforded by the lateral-line system 
of fishes and batrachians, with their marvellous deve- 
lopment to the possession of a sense which we as yet 
scarcely understand, a specification of the sense of pres- 
sure, enabling these lower vertebrates to appreciate cer- 
tain undulations of the water, and thus to detect at a 
distance the presence of objects or the approach of 
changes which under the same circumstances could 
hardly be perceived by the senses of the higher verteb- 
rates. But if the history of evolution be a faithful 
in the history of evolution, without ever having felt 
a sensation. Even the formation of the eye may be 
referred, considering the manner of origin shown by 
the ciliary ganglion and the lens, to the same scheme. 
Originally a number of these sense organs would thus 
seem to have been present in the skin of the verteb- 
rates; but according to this assumption most of them 
have become useless since the development of the senses 
in their present form. The course of this process is 
known at least in the case of one such organ, the pineal 
gland, whose fate has been traced with fair precision. 
a Syst. branch. Sense Org., Quart. Journ. Micr. Sc., Nov., 1885, and Develop. Periplier. New. Syst., same periodical for Oct., 1888. 
b Ueber Anlagen von Sinnesorgo.ncn am Facialis, Glossopliaryngeus und Vagus etc., Arch. f. Anat. u. Physiol. 1885. Anat. Abtheil. 
