362 ZOOLOGY sect. 



The middle ear is absent in the Snakes, though a columella 

 auris is pi'csent, embedded in muscular and fibrous tissue and 

 attached externally, in some cases at least, to the middle of the 

 quadrate. 



Developed in close relation to the epiphysis there is in many 

 Lizards {Laccrtn, Varanns, Anguis, Amphibolurus and others) and 

 in Sphenodon, a remarkable eye-like organ — ihs parietal organ or 

 pineal eye (Fig. 1005), which is situated in the parietal foramen of 

 the cranial roof immediately under the integument, and covered 

 over by a specially modified, transparent scale. The pineal eye is 

 developed from a hollow outgrowth of the roof of the diencephalon 

 in front of the epiphysis; the distal end of this becomes constricted 

 off as a hollow sphere while the remainder is converted into a 

 nerve. The wall of the hollow sphere becomes divergently modi- 

 fied on opposite sides; the distal side gives rise to a lens-like 

 thickening Q), the proximal forms a membrane several layers in 

 thickness — the retina (r.) ; the whole is enclosed in a capsule of 

 connective-tissue (k). The nerve degenerates before the animal 

 reaches maturity, so that the organ would appear — though 

 evidently, from its structure, an organ of sight — to have now 

 entirely or nearly lost its function. 



Reproductive Organs. — The description already given of the 

 reproductive organs of the Lizard (p. 332) applies, so far as all 

 the leading features are concerned, to all the Lacertilia and to 

 the Ophidia ; in Hatteria the copulatory sacs are absent. 



In the Crocodilia and Chelonia, instead of the copulatory sacs there 

 is a median solid penis attached to the wall of the cloaca, and a small 

 process or elitoris occurs in a corresponding position in the female. 

 Though fertilisation is always internal, most Reptilia are oviparous, 

 laying eggs enclosed in a tough, parchment-like or calcified sliell. 

 These are usually deposited in holes and left to hatch by the heat 

 of the sun. In the Crocodiles they are deposited in a rough nest 

 and guarded by the mother. In all cases development has only 

 progressed to a very early stage when the deposition of the eggs 

 takes place, and it is only after a more or less prolonged period of 

 incubation that the young, fully formed in every respect, emerge 

 from the shell and shift for themselves. 



Many Lizards, however, and most Snakes are viviparous, the 

 ova being developed in the interior of the oviduct, and the young 

 reaching the exterior in the completely-formed condition. 



Development. — In all the Reptilia the segmentation is 

 meroblastic, being confined to a germinal disc of protoplasm 

 situated on one side of the yolk. This divides to form a patch of 

 cells which gradually extends as a two-layered sheet, the blasto- 

 derm, over the surface of the ovum. The upper of the two layers 

 is the ectoderm, the lower the yolk-endoderni ; the latter is the 

 equivalent of the mass of yolk-cells of the Frog, and the shallow 



