xm 



PHYLUM CHORDATA 



351 





Reproductive Organs. The description already given of the 

 reproductive organs of the Lizard (p. 323) 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 clitoris 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 

 shell. These are 

 usually deposited in 

 holes and left to 

 hatch by the heat 

 of the sun. In 

 the Crocodiles they 

 are laid 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 vivi- 

 parous, the ova 

 undergoing develop- 

 ment 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-endoderm ; the latter is the 



FIG. 1016. Section of the pineal eye of Sphenodon punc- 

 t a turn, g, blood-vessels ; h, cavity of the eye filled with 

 fluid ; k, capsule of connective-tissue ; I. lens ; m. molecular 

 layer of the retina ; r. retina ; st. stalk of the pineal eye ; 

 x, cells in the stalk. (From Wiedersheim's Comparative 

 Anatomy, after Baldwin Spencer.) 



