CHAPTEE IX. 



REPTILIA. 



THE formation of the germinal layers in the Reptilia is very 

 imperfectly known. The Lizard has been studied in this respect 

 more completely than other types, and there are a few scattered 

 observations on Turtles and Snakes. 



The ovum has in all Reptilia a very similar structure to that in 

 Birds. Impregnation is effected in the upper part of the oviduct, and 

 the early stages of development invariably take place in the oviduct. 

 A few forms are viviparous, viz. some of the blindworms amongst 

 Lizards (Anguis, Seps), and some of the Viperidre and Hydrophidaa 

 amongst the Serpents. In the majority of cases, however, the 

 eggs are laid in moist earth, sand, &c. Around the true ovum an 

 egg shell (of the same general nature as that in birds, though 

 usually soft), and a variable quantity of albumen, are deposited in 

 the oviduct. The extent to which development has proceeded in the 

 oviparous forms before the eggs are laid varies greatly in different 

 species. 



The general features of the development (for a knowledge of 

 which we are mainly indebted to Rathke's beautiful memoirs), the 

 structure of the amnion and allautois, &c. are very much the same 

 as in Birds. 



The Lizards will be taken as type of the class, and a few note- 

 worthy points in the development of other groups will be dealt with 

 at the close of the Chapter. The following description, taken in the 

 main from my own observations, applies to Lacerta mural is. 



The segmentation is meroblastic, and similar to that in Birds. 

 At its close the resulting blastoderm becomes divided into two layers, 

 a superficial epiblast formed of a single row of cells, and a layer 

 below this several rows deep. Below this layer fresh segments 

 continue for some time to be added to the blastoderm from the sub- 

 jacent yolk. 



The blastoderm, which is thickened at its edge, spreads rapidly 

 over the yolk. Shortly before the yolk is half enclosed a small 

 embryonic shield (area pellucida) makes its appearance near the centre 



