26 INVEETEBKATA CHAP. 



The most widespread alteration in the conditions of the larva 

 which is met with is its transformation into an embryo through its 

 retention within the egg-shell or the mother's body. Since, as a race 

 progresses from point to point in evolution, it should, according to 

 the " biogenetic " law, leave behind it a trail of larval stages, each 

 corresponding to a condition of life which had formerly been the 

 adult one, and in each of which the organism would have a distinct 

 method of obtaining its food and a special set of enemies, a very 

 long and complicated life -history should be produced. But the 

 dangers of such a long larval life are very great, therefore a great 

 advantage would be obtained by passing over some of these stages 

 within shelter, and, as was pointed out above, in all life-histories 

 we find an embryonic stage at the beginning. 



Now the food necessary for development during the embryonic 

 phase is, in the vast majority of cases, furnished in the form of yolk 

 platelets, embedded in the cytoplasm. This yolk sometimes distends 

 the embryonic cells to enormous proportions ; it renders the process 

 of cell-division difficult, and sometimes even impossible. In the 

 ordinary process of cell-division each daughter nucleus becomes at, 

 and immediately after the time of nuclear division, the centre of an 

 attractive force which tends to collect the cytoplasm round it like 

 a ball. In some cases, in consequence of this force, the first products 

 of the division of the egg appear as spheres touching one another 

 only at a point. But this period of activity is succeeded by a period 

 of quiescence, and" the centripetal force subsides, so that the 

 cytoplasm of the two daughter cells tends to flow together again, 

 unless a cell membrane has been formed between them in the 

 meantime. 



When yolk is present it impedes the action of the centripetal 

 force, apparently by rendering the cytoplasm less viscous ; for 

 cytoplasm devoid of yolk behaves like thick honey, whilst that 

 which is loaded with yolk behaves more like a mixture of honey 

 and water. Consequently, in yolky eggs we find that in the first 

 stage of their development they either divide into a few large clumsy 

 cells, or else that cell division is represented by nuclear division only. 

 Further, since the yolk is never uniformly distributed in the ovum, but 

 is usually massed at one side, the first divisions result in the produc- 

 tion of cells of unequal sizes or in the production of a cap of cells at 

 one side of the egg, the rest remaining unsegrnented. The pole of the 

 surface of the egg which is relatively freer from food-yolk, and where 

 the division into cells first occurs, is termed the animal pole of the 

 egg ; it is here too that the polar bodies are given off. The opposite pole, 

 where most of the yolk is accumulated, is termed the vegetative pole. 



Such eggs are termed meroblastic, whilst eggs in which the yolk 

 is sufficiently small in amount to allow of the division of the whole 

 are said to be holoblastic. Further, whilst in most eggs the yolk is 

 massed at one side (telolecithal) (vide Fig. 3 A), in a wide range of eggs 

 it is massed at the centre, surrounded by a rind of comparatively yolk- 



