202 



PRINCIPLES OF ANIMAL BIOLOGY 



liquid-filled cavity in the interior. The whole embryo is now designated 

 a hlastula, the cavity within it the hlastocoele. 



In telolecithal eggs, cleavage is considerably modified. In general, 

 the third cleavage is elevated toward the animal pole, so that the upper 

 quartet of cells is smaller than the lower. Also the divisions occur 

 earlier and require less time near the animal pole than at the vegetative 

 pole, with the consequence that the smallness of the upper cells is accen- 

 tuated. In some way connected with this difference between the poles, 

 the blastocoele is eccentric in position, nearer the animal pole. All 

 these features are sho^vn in the frog cleavage (Fig. 172, second row). 



In fishes, reptiles, and birds there is so little protoplasm in the yolk- 

 laden vegetative part of the egg that no cleavage occurs there at any stage. 

 Only the cap of protoplasm above the yolk segments and the blastocoele 

 is bounded by a layer of cells above and by undivided yolk below (Fig. 

 172, third row). In the bird egg in the figure the animal pole is in the 

 center of the first three illustrations, but at the top in the fourth. 



Fig. 173. — Cleavage in arrowworm Sagitta, showing x body (x), which identifies the germ 



cells. 



In insects, cleavage is limited to the surface of the egg, where most 

 of its protoplasm is located. A layer of cells is formed there (Fig. 172, 

 below), while in the interior of the egg is undivided yolk. There is no 

 hollow interior corresponding to the blastocoele at this stage in the insect 

 egg. 



First Differentiation during Cleavage. — Later stages of embryonic 

 development are replete with diffenniiiations of cells. Far in ad\'an(;e 

 of them is a most important differentiation, that between sterile cells 

 which go to form the body {somatic cells) and those which retain their 

 reproductive powers and give rise to the germ cells. In some animals 

 this distinction arises during cleavage, ev(Mi in Acry early cleavage. 



In the arrowworm Sagitta the egg contains a small object, the x body, 

 which in the first six divisions goes undivided into one of the cells (Fig. 

 173). Thus in the 64-cell stage only 1 coll contains an x body. This is 

 the forerunner of all the germ cells, the other 63 are somatic cells. After 

 the sixth cleavage, the x body divides at each cell division, and every 

 germ cell contains it. 



