viii DEVELOPMENTAL ADAPTATIONS 



expanded outer ends of the two external gills. The two membranes 

 so formed, one on each side of the l>n<ly, ;uv closely applied to tin- 

 inner surface of tin- thin egg-envelope. The outer surface of the 

 envelope is in turn in intimate contact with the highly vascular 

 lining of the pouch which sends projecting folds in between the eggs. 

 We have here clearly an adaptive arrangement to minister to the 

 respiratory needs of the developing young, analogous with that pro- 

 vided by the allautois of a Eeptile or Bird. 



It appears somewhat puzzling that the eggs of Nototrema should 

 come to be contained in a pouch the opening of which is much 

 smaller than the cross-section of the egg. The probability appears 

 to be (Boulenger, 1895) that the pouch is formed in response to the 

 presence of the eggs upon the animal's back, a ridge growing up 

 round the eggs as in the case of Hyla goeldii but in this case con- 

 tinuing its growth towards the mesial plane until the corresponding 

 upgrowths from the two sides meet and completely roof in the pouch- 

 like cavity. Support is given to this explanation by the condition 

 in N. pygmaeum where the opening of the pouch is in the form of a 

 median longitudinal slit, prolonged forwards as a kind of seam or. 

 raphe along which the roof of the pouch readily tears and which 

 presents all the appearance of having been formed by the com in;: 

 together of two originally separate lips. 



It must never be forgotten that such peculiarities of development 

 as have been alluded to in the above-mentioned Anura involve 

 adaptive modifications on the part of the young individual itself. 

 The most frequent of such modifications is physiological adaptation, 

 as shown for example by the fact that the transference of the young 

 individual to water before the normal time is commonly fatal. In 

 other cases structural adaptations of a more conspicuous kind are 

 apparent. Thus in Pipa the late tadpole stage, although enclosed 

 within its cell, develops a broad and highly vascular tail which 

 doubtless serves for respiratory and possibly nutritive interchange 

 with the maternal tissues : again in Nototrema the external gills 

 show the peculiar modification already alluded to. The true external 

 gills are in several cases absent, their function being taken over 

 by the vascular surface of the yolk, while in such a case as Rana 

 opisthodon special new respiratory organs have been developed. 



In the various modifications of development dealt with in the 

 preceding section we have to do with attempts, so to speak, on the 

 part of isolated members of a particular group of Vertebrates (Anura) 

 to lessen the degree of their dependence upon the ancestral aquatic 

 habitat. Such attempts amongst the existing Amphibia are not 

 altogether successful: the group as a whole remains chained to a 

 watery, or at least humid, cnvinmment. 



The lower Vertebrates which made a real success of terrestrial 

 existence, emancipating themselves entirely from the aquatic environ- 

 ment, are represented to-day by the Anmiota, and it remains now to 

 study their special modifications of development. 



