138 Heredity and Environment 



each other it is probable that they are passively carried about by 

 currents in the cytoplasm ; the entrance of the sperm serves as a 

 stimulus to the egg cytoplasm which moves according to its pre- 

 established organization. 



2. Cleavage and Differentiation. When the sperm nucleus 

 has come close to the egg nucleus the sperm centrosome usually 

 divides into two minute granules, the daughter centrosomes, which 

 move apart forming a spindle with the centrosomes at its poles 

 and with astral radiations running out from these into the cyto- 

 plasm (Figs. 4, 42 F, 43 B-E). 



Egg and Sperm Chromosomes. At the same time the chro- 

 matin granules and threads in the egg and sperm nuclei take the 

 form of chromosomes, and at this stage it is sometimes possible 

 to see that each chromosome is composed of a series of granules, 

 like beads on a string; these granules are the chromomeres 

 (Fig. 4 L). The number of chromosomes is constant for every 

 species and race, though the number may vary in different spe- 

 cies. In the thread worm, Ascaris megalocephala, there are 

 usually two chromosomes in the egg nucleus and two in the 

 sperm nucleus (Fig. 43 D). In the gastropod, Crepidula (Fig. 

 45), there are about thirty chromosomes in each germ nucleus and 

 sixty in the two. 



Distribution of Chromosomes. Then the spindle and asters 

 grow larger and the nuclear membrane grows thinner and finally 

 disappears altogether, leaving the chromosomes in the equator 

 of the spindle (Figs. 5 A, 6 F, 42 F, and 43 F). Each of the 

 chromosomes then splits lengthwise into two equal parts, and in 

 the splitting of the chromosomes it is sometimes possible to see 

 that each bead-like chromomere divides through its middle. The 

 daughter chromosomes then separate and move to opposite poles of 

 the spindle, where they form the daughter nuclei, and at the same 

 time the cell body begins to divide by a constriction which pinches 

 the cell in two in the plane which passes through the equator of 

 the spindle (Figs. 5, 7, 43 F, 45 B). Finally the chromosomes 



