FORMATION OF THE EMBRYO. 



147 



nal ovum, but it has now become a dual being which has resulted 

 from the fusion of two cells, and it is now capable of develop- 

 ment into a new individual. The process of fertilization appears 

 to be essentially the same among all higher animals, and in a 

 broader sense to be identical with the sexual process among all 

 higher and many lower plants. (Compare the fern, p. 98.) 



Cleavage of the Fertilized Ovum. Soon after fertilization the 

 ovum begins the remarkable process of multiplication which has 

 already been briefly sketched at p. 51, Chap. IV. The segmen- 

 tation-nucleus divides into two parts, and this is followed by 

 a division of the vitellus, each half of the original nucleus 



ec 



FIG. 84. Diagram of the early stages in the development of the earthworm ; a, the bias- 

 tula in surface view; 6, the same in section showing the large segmentation-cavity; c, 

 beginning of the invaginatiou to form the gastrula; d, the gastrula in section ; e, section 1 

 of the gastrula along the straight line (ss) shown in d ; /, older gastrula showing the 

 growth of the mesoblast; ec, ectoblast; en, entoblast; m, m 1 , primary mesoblastic cells; 

 Mt 2 , mesoblast. 



becoming the nucleus of one of the halves of the vitellus ; that 

 is, the original cell divides into two smaller but similar cells (see- 

 Fig. 21). These divide in turn into four, and these successively 

 into eight, sixteen, etc., until the embryo consists of a large num- 

 ber of cells. At the division of the four cells into eight, four are 

 smaller and four larger, so that an upper and a lower pole may 

 be distinguished in the embryo. These poles can be recognized 

 throughout the remainder of this period of development, the cells 

 of the upper half of the embryo remaining smaller and less gran- 

 ular than those of the lower half. 



Almost from the first the cells arrange themselves so as to 



