REPRODUCTION 



39 



'I'he smallest egg cells are large compared to most tissue cells of the 

 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, a substance which ^^ 



undoubtedly plays an important part 

 in determining the course of develop- 

 ment. 



In Amphioxus. Amphioxus is not 

 literally a vertebrate. But it is a 

 chordate and in many respects obvi- 

 ously primitive. The adult is a 

 slender fish-like animal about 5 cm. 

 long (Fig. 11). The egg is correspond- 

 ingly small, about o.i mm. in diam- 

 eter, and contains very little yolk. 



(Fig. 32) 



The plane of the first cleavage 

 (Fig. 33) of the egg corresponds to the 

 definitive median (sagittal) plane of 

 the future adult. The two cells re- 



VEG. 

 Fig. 32. — Median section of a ferti- 

 lized egg of AMPHIOXUS. Diameter 

 of egg about 0.1 mm. ^ TV, animal pole; 

 suiting from the first cleavage there- iV, male and female pronuclei ;P, polar 

 r J. J.I. • Ui J 1 rj. body; S, remnant of spermatozoon; 



fore represent the right and left y^^, vegetal pole; Y, region of cyto- 



halves of the body. The plane of the plasm occupied by coarse granules of 

 J , . J- ^ J. yolk. (After Cerfontaine.) 



second cleavage is perpendicular to 



that of the first and the third cleavage plane is perpendicular 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. 

 Meanwhile 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 thirty-two cell stage the cells are disposed to form a hollow 

 sphere whose wall is everywhere one cell in thickness. Thus every cell 

 of the thirty-two is in direct relation to the exterior, a most favorable 

 position for respiration and excretion. This hollow spherical shape is 

 retained as cleavage continues (Fig. t^^ G-I) until between two hundred 

 and three hundred 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 

 the resulting cells. This inequality persists as cleavage goes on. It is 



