PEOPLING OF LAND AND SEA 179 



leaves of water-side trees on which they live. The young are 

 hatched within this sticky envelope, which the rains soak and 

 carry away into the stream below, together with the young 

 Batrachians inside. A Tree-frog of the Antilles, Hylodes 

 martinicensis, goes even further than this ; it also attaches its 

 eggs to the undersides of the leaves on the trees it inhabits, but 

 the young are born in their permanent form. The same is true 

 of Rana opistliodon and Hyla nebidosa. These are still but 

 tentative experiments, so to speak, in passing from an aquatic 

 or amphibian life to the life of the open air. Moreover, these 

 experiments, favoured by the large size of the eggs, very rich 

 in reserve substances, are relatively recent, since the tailless 

 Batrachians of the Frog type date back no farther than the 

 Tertiary Period, and there were Reptiles definitely terrestrial 

 in their habitat at the end of the Primary. In Tertiary times 

 the Batrachians renewed, this time without complete success, 

 the happy attempt of more remote times, to become dwellers 

 in the free air. However, the success of the earlier efforts had 

 been assured by a new phenomenon : nutritive material had 

 become accumulated within the egg, which finally became of 

 considerable size. Thus the embryo, finding within the egg all 

 the nutriment necessary, and not being compelled to expend 

 any of its energy in the search for food, employed it in 

 multiplying its structural cells. Hence its development was 

 considerably accelerated. The phases of development corre- 

 sponding to the aquatic life were gradually curtailed, finally be- 

 coming mere indications. Since the limbs remained unused they 

 were superseded in growth by the viscera and nervous system, 

 and the external form of the embryo was thus temporarily modi- 

 fied, the more so as it was obliged to expand temporarily above 

 the vitelline substance, which had become tremendously en- 

 larged, and to take on the appearance of a plate formed of three 

 superimposed layers, corresponding to the ectoderm, the meso- 

 derm, and the endoderm. Furthermore, those portions of the 

 embryo destined to disappear before birth have been constituted 

 by some of the cells born of the segmentation of the nucleus and 

 of the vital plasma of the egg, which have spread around the 

 spherical yoke mass and gradually enveloped it, producing the 

 blastoderm, which consists of cells that retain their primitive 

 aspects and merely fulfil a digestive function. This was 

 probably due to the fact that the first cells formed were early 



