458 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



numerous cases the natural selection value of the colouring as a 

 means of making the egg less conspicuous is obvious. 



In Eeptiles the tertiary envelopes resemble those of Birds though 

 in many cases, as in various' Lizards and Turtles, there is no definite 

 rigid shell. On the other hand there may be a certain amount of lime 

 deposited in the outer layers of the shell -membrane. The albumen 

 varies in amount : in Sphenodon it forms only a very thin layer (Dendy, 

 1899). 



In Elasmobraneh fishes the egg is again enclosed in a layer of 

 albumen and this in turn surrounded by a shell. The shell is of a 

 horny consistency and is frequently rectangular and pillow-shaped. 

 Characteristic differences are found in different genera and species. 

 Thus in the Skates (fiaia) each angle is prolonged so that the egg 



Fig. 207. — Egg of Scylliiim, held in position by its four elastic filaments being wound 

 round a plant. (Figure by Kopsch, from The Cambridge Natural History.) 



has an outline like that of a hand-barrow. In Scyllium (Fig. 207) 

 the prolongations become long spirally coiled anchoring filaments : 

 in Pristiiirus two short prolongations occur at one end while the 

 other end is simply rounded. 



II. Modifications of the Envelopes and other Adaptive 

 Modifications occurring during the Early Development of the 

 Amphibia. — The Amphibians form a group of Vertebrates which 

 have taken less or more completely to a terrestrial existence in their 

 adult condition. They have not been able to emancipate themselves 

 entirely from the ancestral aquatic habitat, possibly on account of 

 the feeble development of the horny outer layer of the epidermis. 

 They are still as a rule entirely aquatic during the early stages of 

 their development, the eggs being laid in water and the young animal 

 passing its larval existence in the water. 



In a number of cases, particularly in Anura inhabiting tropical 

 regions with a well-marked dry season, very interesting adaptations 

 are found whereby the young animal is enabled to pass a more or 

 less prolonged period out of the water. In the first type of these 



