THE DEVELOPMENT OF ORGANISMS 719 



The young ovum is sometimes slightly amoeboid; and occasionally, 

 as in Hydra, there is an exaggeration of this feature. In the fresh- 

 water Hydra, as in the simple marine hydroid Tubularia and some 

 other cases, the young ovum engulfs the adjacent cells in the ovary, 

 so that there is a very drastic struggle for existence at the very 

 threshold of life. In the case of Hydra, a single ripe ovum is the only 

 vigorous cell left in the minute ovary. But in most cases the nutritive 

 material is obtained more passively, e.g. from special yolk-glands, 

 as in Flat Worms, or from the vascular fluid of the body, as in birds 

 and reptiles. The yolk often consists of a mixture of fatty sub- 

 stances, like lecithin, and protein substances, like viteUin. Since the 

 yolk, whether large or small in amount, is intracellular and sur- 

 rounded by the cell-membrane or vitelline membrane, it must not 

 be confused with the more external "white of egg" which is often 

 present. This albumen, familiar in the eggs of birds and in the jelly 

 of frog's spawn, is partly nutritive and partly protective. It is 

 usually a proteinaceous secretion of the waU of the oviduct. 



Of some importance is the amount and disposition of the yolk 

 within the egg, for this largely determines the way in which the 

 fertilised egg-cell will divide, {a) The yolk may be small in amount 

 and distributed uniformly, as in the ova of ordinary mammals, 

 earthworms, starfishes, and sponges. In such cases the segmentation 

 is total (holoblastic) and equal, (b) Or the yolk may be more 

 abundant and sinks to the lower hemisphere of the ovum, being 

 heavier than the living matter. This is well seen in the amphibian 

 egg, and it also occurs in some sponges, some Coelentera, worms, 

 and molluscs, in the lamprey, and in Ganoid and Dipnoan fishes. 

 There is not enough of yolk to hinder complete segmentation, but 

 the cells are unequal, the upper hemisphere showing more numerous 

 smaller cells, the lower hemisphere showing fewer, larger, yolk- 

 laden cells. This is called holoblastic unequal segmentation, (c) A 

 third arrangement is seen in many crustaceans and insects, but not 

 outside the Arthropod phylum. There is a considerable quantity of 

 yolk, which accumulates in the core of the spherical or oval egg-cell, 

 and is entirely surrounded by the formative protoplasm. This 

 results in partial (or meroblastic) and peripheral segmentation, 

 the nutritive core remaining undivided, {d) Fourthly, there may be 

 a relatively large quantity of yolk on the top of which the formative 

 protoplasm lies as a small polar disc, like a watch-glass turned 

 upside down. Only the polar area divides, forming a disc of cells or 

 blastoderm. The mass of (telolecithal) yolk is purely nutritive, 

 feeding the embryo, but not forming any of it. This is the partial 

 discoidal type of segmentation, and it occurs in all Cephalopods, all 

 cartilaginous and Teleostean fishes, all reptiles and birds, and also 

 in the two lowest mammals — the oviparous Monotremes, the duck- 

 mole and the Spiny Ant-eater. We shall return to the segmentation 



