Reproduction 



247 



animal to which the egg belongs. The metabolic rate in an egg before 

 fertilization is relatively low. After fertilization the rate increases. 

 Before entering upon a prolonged period of activity at high metabolic 

 rate, the bulky ovum increases its surfaces by dividing into small 

 cells — the process called cleavage. The successive divisions of the 

 original egg nucleus are, in fact, accompanied by absolute increase in 

 the quantity of nuclear chromatin, an 



a substance which undoubtedly 

 plays an important part in deter- 

 mining the course of development. 



In Amphioxus. Amphioxus is 

 not literally a vertebrate. But it is 

 a chordate and in many respects 

 obviously primitive. The adult is a 

 slender, fishlike animal about 5 cm. 

 long (Fig. 313). The egg is corre- 

 spondingly small, about 0.1 mm. 

 in diameter, and contains very 

 little yolk (Fig. 204). 



The plane of the first cleavage 

 (Fig. 205) of the egg corresponds 

 to the definitive median (sagittal) 

 plane of the future adult. The two 

 cells resulting from the first cleav- 

 age therefore represent the right 

 and left halves of the body. The 

 plane of the second cleavage is per- 

 pendicular to that of the first, and 

 the third cleavage plane is per- 



Fig. 204. Median section of a ferti- 

 lized egg of Amphioxus. Diameter of 

 egg about 0.1 mm. (AN.) Animal pole; 

 (N) male and female pronuclei; (P) 

 polar body; (S) remnant of spermato- 

 zoon; (VEG.) vegetal pole; (Y) region 

 of cytoplasm occupied by coarse 

 granules of yolk. (After Cerfontaine. 

 Courtesy, Neal and Rand: "Chor- 

 date Anatomy," Philadelphia, The 

 Rlakiston Company.) 



pendicular to both the first and 

 second. The second and third cleavages each divide the egg slightly 

 unequally. Further cleavages follow one another in rapid succession, 

 their planes adhering to a fairly rigidly determined order. Mean- 

 while the cells gradually shift their relative positions and surfaces 

 of contact in such a way that a space opens out at the center of 

 the whole mass. At the 32-cell stage, the cells are disposed to form 

 a hollow sphere whose wall is everywhere one cell in thickness. Thus 

 every cell of the 32 is in direct relation to the exterior, a most favorable 

 position for respiration and excretion. This hollow spheric shape is 

 retained as cleavage continues (Fig. 205G-I) until between 200 and 

 300 cells have been formed. This stage of the embryo is called the 

 "blastula." The name blastocoele is applied to the cavity. 



The second and third cleavages introduce inequality of size among 



