VERTEBRATA. 



343 



in tendons. Such bones are known as sesamoids. The most familiar 

 example is the patella or knee-pan. 



The nervous system is divisible into central and peripheral parts. The 

 former is constituted by the brain and spinal cord ; the latter by the 

 nerves and sympathetic system. The brain consists in the embryo of 

 three hollow vesicles a fore-, mid-, and a hind-brain. The first of these 

 eventually gives origin to the two olfactory lobes (rhinencephala), the 

 two cerebral hemispheres (prosencephala), whilst a remnant of the original 

 vesicle persists as the thalamencephalon or vesicle of the thalami optici, 

 from which arise the two hollow outgrowths converted afterwards into the 

 optic nerves, retina and retinal pigment layer. The pineal gland (epiphysis 

 cerebri) is continuous with the roof of the thalamencephalon. It has been 

 supposed to represent either the region of closure of the neural folds 

 (cf. Cephalochorda^} or else, an unpaired eye 1 . The floor of the thalamen- 

 cephalon is prolonged into a hollow infundibulum, to the apex of which 

 is attached the pituitary body (hypophysis cerebri = conarium), derived 

 in part from the ectoderm of the stomodaeum, in part from mesoblast. 

 It is supposed to represent either the sub-neural gland of Urochorda 

 (Balfour, Julin) or an abortive pair of gill-clefts (Dohrn). The floor of 

 the mid-brain (mesencephalon) becomes crura cerebri ; its roof simple 

 in Protopterus and Siredon ( = Axolotl) forms two hollow optic lobes 

 or corpora bigemina ; or in Mammalia four solid corpora quadrigemina. 

 The notochord in the embryo extends as far forwards as the mid-brain. 

 At one period the whole fore-brain is bent or deflexed more or less in 



1 In Cephalochorda and Urochorda the neural folds unite over the groove from behind for- 

 wards, and a pore opening into the brain-cavity persists anteriorly for some time. In Vertebrata, 

 however, the folds unite from before backwards. Gotte describes in the embryo Bombinator a cord 

 of cells uniting the pineal body with the epidermis, and suggests that in this union is to be seen 

 a remnant of the pore alluded to above. A short slit has been found by Van Wijhe in the same 

 position in the embryo Elasmobranch. Recent researches have shown that in Lacertilia the apex of 

 the pineal gland is transformed into an azygos eye. According to de Graaf (Z. A. ix. 1886), this 

 eye, which is detached from the pineal gland itself, lies below the parietal foramen of the skull, 

 between the membranes of the brain. It is a vesicle with cellular walls. The part of the wall 

 turned towards the foramen is thickened and lens-like ; the remaining part is deeply pigmented, and 

 in Anguis fragilis has a complex structure, consisting of three tiers of cells. Of these, the one 

 turned towards the cavity of the vesicle has its cells elongate, pigmented and provided with refractile 

 processes. W. B. Spencer (cf. Nature, xxxiv. 1886) has confirmed and extended de Graaf s results. 

 He finds that the azygos eye is sometimes embryonic (?), e.g. in Cydodus, sometimes highly developed, 

 e.g. in Hatteria and Iguana. He finds that it is often connected to a nerve, which may be traced 

 into the pineal gland, and is always accompanied by a blood-vessel ; that the pigmented cells are 

 transversely striated, and appear to be connected at their bases with the cells of the two other tiers, 

 the whole forming a complex retina ; that the cavity of the vesicle is filled with a coagulable fluid. 

 But he has not observed the refractile processes of de Graaf. Mr. Spencer will publish his results 

 in extenso in the Q. J. M. Ahlborn came to the conclusion, on various grounds, that the pineal 

 gland represents a rudimentary eye (Z. W. Z. xl. 1884), but the view appears to have been also 

 hinted at by Rabl-Riickhard (see, Z. A. ix. 1886, p. 405). The extinct Stegosauria possess a large 

 parietal foramen, which, as suggested by de Graaf, may possibly indicate the former existence of a 

 pineal gland of great functional importance. 



