I'INKAL (;LAM>. 



li: 



the Lainjjreys as tlie liiisis of liis explanation of tlie eorres- 

 [lunding struetiires in the higher \ertel5rates: — and even he 

 could declare that "tlie 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 Beakd" 

 drew a picture of tlie worm-like ancestors of the \'er- 

 tebrates, ancestors \\ liicli liave all disappeared from the 

 earth, so far as a\c know, without leaving a trace of 

 their existent-e. Their body, he says, was anteriorly 

 furnished Avith several, at least eleven pairs of gill-clefts, 

 and above, i. e. at the dorsal termination of each gill- 

 cleft, la}" a sense organ in the skin, probablj- to w-arn 

 the animal of the approach of any dangerous object 



guide, sense organs of the same kind have originated 

 tlie development of all the other higher senses. Even 

 in the mammals Fkoiuep'' has traced the same course 

 of development for three of the cranial nerves (faciaUs, 

 (/lossophari/ngeus, and vagus) as Beakd found to obtain 

 in Kays and other fishes for all these nerves, including 

 tliose of smell, liearing, 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. Tiic ganglion is form- 

 ed by the growth and multiplication of the epidermal 

 cells at this point; but the rudiment of the primordial 



towards the gill-bearing parts of the body. The nearest ! sense organ disappears after having thus done its duty 



C 



L.ol. 



r]/;Mri p^l // 



VII 17 m\ \ CI, 



Fig. 347. Tlirec sketclies of the brain iiml mcdtill.'i obloug.ala in a Pride (Petroint/zori in the Ammoca'tes stage), magnified. After ^VIEDERS- 



HEIM. A, from below; B, from the right; C, from above. 



/ — A", cranial nerves in order from in front; 1, first pair of spinal nerves; Cb, cerebrum; Obi, cerebellum; Ep, epiphysis; Hi/p, hypophysis; 



L.ol., lobi olfactorii; ME, mesencephalon; Med, medulla; Mob, medulla oblongata; S. v., saccus vasculosus; Tlit, thalamencephalon. 



jiarallel 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 j^et 

 scarcely understand, a specification of the sense of pres- 

 sure, enaliling 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 lie 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 \vith fair preci.sion. 



Si/st. branch, ^iense Org., Qnart. Journ. Micr. Sc, Nov., 1885, and Develop. Peripher. Nerv. Syst., same periodical for Oct.. 1888. 

 Ueber Anlagen von Sinnesorganen am Facialis, Glossopharijngeus unci Vagus etc., Arch. f. Anat. u. Physiol. 1885. Auat. Abtheil. 



