MECHANISM IN SEGREGATION 43 



is that two cells are produced, one with six, the other 

 with five chromosomes (Fig. 17, d). 



Without a resting stage a new spindle develops in each 

 cell, and a new division takes place — each dumb-bell- 

 shaped body dividing, as well as the sex chromosome in 

 the cell that contains it. In all, four cells result (Fig. 17, 

 e and /) — two with five chromosomes each, two with six 

 each. Each becomes a spermatozoon, which in this worm 

 is a round cell with the chromosomes at one pole (Fig. 

 17, g). Half of the spermatozoa contain six, half 

 five chromosomes. They can be distinguished even in 

 the living sperms (Fig. 17, h). If a six-chromosome 

 sperm fertilizes an egg (Fig. 16, b), a female (with 12 

 chromosomes) is produced — if a five-chromosome sperm 

 fertilizes an egg (Fig. 16, c), a male (with 11 

 chromosomes) is produced. 



The two chromosome divisions (or separations) that 

 take place when the polar bodies are extruded from the 

 egg are, for a number of reasons that need not be entered 

 into here, generally regarded as equivalent to the two 

 final divisions in the ripening of the sperm-cells. One 

 of the two divisions is interpreted as an ordinary cell- 

 division in which the chromosomes split lengthwise into 

 equivalent halves — half going to each pole. The other 

 division is interpreted as a separation of whole chromo- 

 somes that have come together side by side at an earlier 

 stage. The tetrad is, then, looked upon as a pair of 

 chromosomes that have conjugated in the sense that they 

 have come to lie side by side (with interchange of mate- 

 rials at times in a way to be described later). One split 

 is supposed to correspond to the line between the conju- 

 gated pairs ; the other split represents a division in each 

 chromosome of the pair. As a consequence when the 

 chromosomes move apart (at the maturation division) one 

 of the two divisions is said to be a *^ reducing division** 

 because whole chromosomes are supposed to separate ; the 

 other division is said to be an ^^ equation di\dsion,** each 



