G34 



ANATOMY OF VERTEBRATES. 



444 



in which the optic nerves chiefly originate : the ventricles are 

 large in each mass. The eye-ball is formed, as in Fishes, by the 

 bending of a sausage-like bag about the lens, and the coalescence 

 of the ends brought into contact. The cicatrix, shown in fig. 443, 

 soon disappears. The capsule is next differentiated from the lens 

 proper. The eye-ball is, at first, unprotected, as in Fishes ; but 

 the contiguous skin -border begins to encroach upon its fore part, 

 with modified growth, to form the eyelids. 



After the developement of the labyrinth from the primitive 

 ear-cajjsule, a tympanic cavity is formed, in which the ' stapes ' 



appears as a short thick cartilaginous 

 cylinder in the Chelonia, in which the 

 ( meatus auditorius ' broadens outwards 

 to a trumpet shape, which it retains. In 

 the Ophidia the ' stapes,' fig. 444, b, e, is 

 similarly developed, independently of the 

 tympanic and mandibular (or so-called 

 Meckel's) cartilage, ib. d, as shown by 

 Rathke, in Natrix torquata. The nostrils 

 appear as deep depressions at the fore end 

 of the head, the margins of winch become 

 incurved, and the bottom of the sac is 

 produced into a canal communicating with 



embryo fa Q mQui \ lm 



In and from the membrane of the 

 notochord, continued along the basis cranii, is developed the 

 eartilage of the basi-presphenoid, blending laterally with the 

 ear-capsules : the basal cartilage bifurcates anteriorly, and re- 

 unites surrounding the hypophysial fissure : it is then continued 

 singly forward, and expands anteriorly in connection with car- 

 tilaginous plates from which, in Chelonia and Ophidia, the 

 profronto-nasal bones are developed. In Lacertilia the large 

 ' lacrymal ' bones grow from the same embryonal cartilage. 

 Behind these foremost representatives of neurapophyses are 

 three pairs, more clearly showing the neurapophysial characters : 

 the pair resting on the bifurcated presphenoidal part of the 

 basal cartilage, in relation to the optic nerves, become ' orbito- 

 sphenoids ; ' l the next pair, in relation to the trigeminal nerves, 

 situated anterior to the ( ear-capsules,' become the ' alisphe- 

 noids;' 2 the pair behind the ear-capsules, resting on the 

 basioccipital, become the ex- and par-occipitals. 3 Thus the 



1 cccxxx. taf. vii. fig. 17,/, ' vordcrcr Keilbeinflugel.' 2 Ib. e, ' hintercr Keilbeinflugel. 

 3 lb. b, ' Seitentheil dcs Hinterhauptbeines.' 



Haemal arches of cranium 

 Suake. cccxxx. 



