It is worth while to pause here to consider how these early changes are assisted by the peculiar condition of the egg at the time of laying. The southern hemisphere of the egg is laden with a store of food. The food is dead, and acts as a mechanical hindrance to the activity of the living matter. In the northern half of the egg but little food is present to impede its activity, and it is plainly important that this half shall receive as much warmth as possible from the uncertain sunshine of early spring. Two circumstances ensure this. In the first place, the food-laden region is the heaviest part of the egg, so that the latter—buoyed up as it is by the jelly—tends to float with its most “alive” part upwards. Secondly, this upper part is coloured black,—a great advantage, since black objects absorb heat readily. Both these peculiarities therefore favour the more rapid development of the “northern” hemisphere. Hence the third cleft is to the north of the egg’s equator, instead of being halfway between the upper and lower [Pg 344] poles, and hence, too, at the close of segmentation the northern segments are smaller and more numerous than the southern. In the case of the hen’s egg (Chapter XVI.), the amount of stored food contained in the egg (the yolk) is so enormous that segmentation is confined entirely to a small patch on the upper surface. Another result of the relatively small amount of stored food in the frog’s egg is that the tadpole is compelled to turn out and earn its own living at a stage when the chick’s inherited fortune is still considerable. The tadpole.—The spherical mass soon becomes ovoid, and is divided into head and trunk by a neck-constriction (J). An occasional wriggle shows that the creature is alive. Shortly afterwards a tail grows out from the hinder end of the trunk, giving the animal something of the appearance of a fish (L). In this stage the tadpole makes its way out of the jelly, and thus hatches, about a fortnight after the laying of the eggs. The little creature is quite helpless; it has no mouth (the egg food is not yet exhausted, however), and its attempts at swimming are still feeble and uncertain. In this defenceless condition (Fig. 219, 1) it will be seen to attach itself to the water-weed of the aquarium by means of a sucker (Fig. 218, L, sk) on the underside of its head. It is in a very favourable position for examination, and by help of a lens, two—soon there are three—pairs of fine, thread-like outgrowths can be distinguished on the sides of the neck (Fig. 219, 2 and 2a). These are the external gills. In a few days the mouth breaks through, and the animal begins to nibble at the vegetation with little horny jaws, and soon swims about the aquarium with confidence. Fig. 219.—The Frog. Stages in the life-history, from the newly-hatched Tadpoles (1) to the young Frog (8). (× 1.) 2a is a magnified view of 2.