REPRODUCTION 



59 



are respectively equal to the combined volumes of the corresponding 

 parts of the two cells because the Hmiting membranes of the two cells 

 have greater total area than those of the single cell. 



The 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 

 fertiUzation is relatively low. After fertiUzation the rate increases. 

 Before entering upon a prolonged ^^^^ 



period of activity at high metabolic 

 rate the bulky egg-cell increases its 

 surfaces by dividing into small cells. 

 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 impor- 

 tant part in determining the course 

 of development. 



In Amphioxus. From the fore- 

 going it follows that the first necessary 

 process in development is the cleavage 

 of the fertilized egg (Fig. 36) into yeg. 



many small cells. Figure 37 illus- Fig. 36. — Median section of a ferti- 



, , ,, r , • lized egg of AMPHIOXUS. Diameter 



trates the course of cleavage m of egg about o.i mm. ^iV, animal pole; 



Amphioxus. Although sometimes A/', male and female pronuclei; P, polar 



, .J- J 1 A 1- • • i. body; 5, remnant of spermatozoon; 



classified as such, Amphioxus is not ^^c. vegetal pole; Y, region of cyto- 



literally a vertebrate. But it is a plasm occupied by coarse granules of 



, 1 , , . , , . yolk. (After Cerfontaine.) 



chordate and in many respects obvi- 

 ously primitive. The adult is a slender fish-like animal about 5 cm. long 

 (see Fig. 14). The egg is correspondingly small, about o.i mm. in 

 diameter, and contains very little yolk. 



The plane of the first cleavage of the egg corresponds to the definitive 

 median (sagittal) plane of the future adult. The two cells resulting from 

 the first cleavage therefore represent the right and left halves of the body. 

 The plane of the 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 



