Page:Popular Science Monthly Volume 59.djvu/88

78 The young that develop from these comparatively large eggs inside this peculiar skin-bag are remarkable enough to satisfy the ideals of so bizarre a parent. Like the young Surrinam toad, these get their front legs at a very early period, and at an early period are also found without the adhesive organs and horny jaws that seem so essential to all common tadpoles. But their chief departure from received tadpole style is the phenomenal character of the gills. These are large, bell-shaped, flower-like membranes that envelop the tadpole like a mantle and, coming between it and the walls of the mother's pouch, may serve as a means of getting oxygen and possibly food from the mother, for the gills are richly supplied with blood-vessels, and the walls of the mother's pouch are also vascular, giving the anatomical conditions for interchange such as takes place in a mammal's placenta. Each of these two gills seems to have been made by the fusing of two specialized gills, and each retains two stalks. Eventually these big gills are probably lost and replaced by inside gills, just as in common tadpoles the outside gills are always followed by inside gills. Whether the mother goes into the water and lets the young escape where they can use their gills, or whether she keeps them at home till they have lost these youthful structures and can 'come out' in budding maturity, is not known, in this case. But in Nonotrema marsupiatum and Nototrema plumbeum the young are set free into the water when they are still tadpoles.

The way in which such a capacious pocket on the back can have come about is perhaps indicated by the state of things in Nototrema pygmæum. The brood pouch is here small and slit-like, and when the young are ready to leave it they press and wriggle till the pouch is torn open, from the external opening forwards. There are only four to seven young that can come forth in this partially Cæsarian way, and their appearance seems to have been prearranged by the way in which the pouch is made. Two folds of the skin grow up to meet one another along the back, and when they fuse they leave a sort of seam, which is also the line along which the pouch ruptures to let the young out. The pouch of this tree-frog is thus to some degree intermediate between the simple cup on the back of Hyla Goeldii, not well shown in Fig. 9, and the more perfect sac of the other pouched frogs described above, and may indicate the lines along which structures and habits like those of Hyla Goeldii could have been evolved in those of Nototrema oviferum. On the other hand, the small size of this frog and the large size of its eggs make it both impossible to get the eggs into the usual external opening of the pouch and impossible for the big larvæ to escape through it; hence it may be that the pouch grows up as folds after the eggs are laid on the mother's back, and that these folds remain easy of separation to allow the young to escape, all independent