462 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



structure which projects forwards between the dorsal surface of the 

 female and the ventral surface of the male. The eggs pass out one 

 by one through this and are distributed at fairly equal intervals over 

 the dorsal surface of the trunk of the female. The skin now pro- 

 liferates actively, growing up so as to form highly vascular partitions 

 between the eggs, each of the latter coming to be enclosed in a 

 deep pit. The mouth of this becomes closed in by a dark-coloured 

 operculum, possibly formed of hardened epidermal secretion. Each 

 egg is thus enclosed in a little chamber in which it passes through 

 the early stages of its development, including a modified tadpole 

 stage, and issues forth eventually (after about 82 days) as a young 

 Toad. 



In another set of Anurous Amphibians the eggs undergo their 

 development in a spacious single cavity 

 within the parental body. In Ehinoderma 

 darwini (Jimenez de la Espada, 1872 ; 

 Plate, 1897) this cavity is the enlarged 

 unpaired croaking sac of the male, into 

 which the eggs, to the number of from 

 5 to 15, are swallowed and from which the 

 young issue after completing the tadpole 

 stage. In the genus Nototrema the brood 

 cavity is a special large pouch lying 

 beneath the skin of the back, lined by 

 involuted epidermis and opening to the 

 exterior just in front of the cloacal aperture. 

 In different species of the genus there is 

 much difference in the length of time 

 during which the developing embryo is 

 retained within the pouch, the length of 

 this period being apparently correlated 

 with the size of the egg and the amount 

 of food -yolk stored within it. Thus in 

 N. marsupiatum there may be as many as 

 200 eggs in the pouch, each measuring 

 about 5 mm. in diameter (Brandes u. 

 Schoenichen, 1901), and the young make 

 their way out as typical tadpoles which doubtless lead for a time 

 a free aquatic existence before metamorphosis takes place. 



In N. oviferum (Weinland, 1854) the eggs are much larger 

 (10 mm.) and fewer in number (about 15) and in this case as in the 

 allied N. testudineum and N. fissipes, which also possess large eggs, 

 the young go on developing within the pouch until after the period 

 of metamorphosis. 



In Nototrema an interesting adaptive feature characterizes the 

 external gills. These organs are present upon branchial arches I and 

 II, each consisting of a long slender stalk, which passes at its outer 

 end into a thin highly vascular membrane formed by the fused and 



Fig. 211.— A, male of Phyllo- 

 bates trinitatis carrying tad- 

 poles ; B, female of Hyla 

 goeldii carrying eggs. (After 

 Boulenger, 1895.) 



