OPERATION OF THE GENETIC SYSTEM 69 



of autosomes; in addition the female has two X's, the male 

 but one X. What difference in development does this make? 



(2) At first the two sexes develop alike, so far as can 

 be seen, so that the individuals are in an "indifferent" con- 

 dition, with respect to sex. 



In this indifferent condition, the embryos develop so far 

 that head, limbs, and all systems of organs are distinguish- 

 able : in the rabbit this condition is said to continue for 

 about 14 days. 



f 



Figure 22. A portion of the germ gland at an early stage in the 

 pig, showing the enlarged genital cells (g). Redrawn from a figure 

 by Nagel, in Hertwig's Handbuch der Entwicklungslehre. 



(3) While in this indifferent condition, certain cells are 

 set off as germ cells. These are gathered into a strip of 

 small cells called the germ gland, lying on the dorsal sur- 

 face of the body wall (figure 22). Some of the small cells 

 become much larger than the others; these are called genital 

 cells. The genital cells are to produce later the new germ 



