vin DEVELOPMENTAL ADAPTATIONS 459 



adaptations we find special modifications of the tertiary envelope 

 which is normally a simple mass of jelly deposited round the egg. 



The first type of such adaptation is exemplified by various species 

 of Hylodes and by Bana opisthodon in which the eggs are simply 

 deposited in free air in damp spots, each surrounded by a transparent 

 spherical protective shell. In B. opisthodon (Boulenger, 1890) the 

 young Frog before hatching develops on the tip of its snout a 

 small conical protuberance apparently used like the egg-tooth of 

 Eeptiles and the similar organ in Birds to tear open the egg-envelope. 

 A further interesting adaptive feature is that the young unhatched 

 Frog possesses on each side of its body a series of vascular flaps of 

 skin somewhat resembling the gill-flaps of an Elasmobranch fish and 

 apparently functioning as respiratory organs. 



In a considerable number of tropical Anura the oviducal secretion 

 which surrounds the eggs is, at the time of laying, beaten up by 

 rapid movements of the hind feet of the parents into a fine foam or 

 froth with numerous entangled air-bubbles. This may be deposited 

 on the surface of a pool where it floats about like a fleck of ordinary 

 foam with the developing eggs scattered through it . (Paludicola 

 fuscomaculata). At a particular stage in development a digestive 

 ferment apparently is secreted, probably by ectodermal gland cells, 

 which liquefies the jelly and allows the larvae to drop through into 

 the underlying water. 1 In other cases the mass of foam is deposited 

 in an excavation in the ground, so situated tha.t rain-water readily 

 trickles into it {Engystoma ovale), or merely in a damp spot. In the 

 case of the Japanese Bhacophorus (Polypedates) schlegeli (Ikeda, 189V) 

 the burrow is made in a bank by the margin of standing water and 

 after the mass of egg-foam has been deposited the pair of Frogs make 

 their way out by excavating a tunnel which slopes downwards and 

 opens near the water's surface. Here again at the appropriate stage 

 of development the jelly liquefies and the young larvae are carried 

 down by it into the water. 



Im the case of Phyllomedusa hypochondrialis the process of ovi- 

 position was observed by Budgett (1899) in the Gran Chaco. The 

 eggs are deposited during the night, the female clambering up 

 amongst the leaves of a suitable plant by the margin of a pool, with 

 the male on her back (Fig. 208). With their hind legs the two Frogs 

 bend the margins of a leaf together so as to form a funnel into which 

 the eggs are poured together with the fertilizing sperm. The eggs 

 are enclosed in a mass of firm adhesive jelly which causes the leaf to 



1 It is probable that such ferments play an important part in softening the egg- 

 envelopes preparatory to hatching in various animals. Thus in Lepidosiren the 

 process of hatching is rendered possible by the softening of the egg-shell brought 

 about apparently by digestive ferment secreted by the ectoderm covering the body 

 (Graham Kerr, 1900). The same appears to be the ease in Teleosts (Wintrebert, 

 1912). In Xenopus amongst Amphibians o, similar process apparently takes place 

 and in this case Bles (1905) attributes the formation of the ferment not simply 

 to the diffuse activity of the ectoderm cells but to the action of a special "frontal 

 eland." It seems not improbable that the formation of such hatching ferments will 

 be found to occur very generally in aquatic Vertebrates. 



