SEX DIFFERENTIATION 303 



is removed the embryonic right ovary will then develop. And it may develop 

 into either a testis, an ovary, or an ovotestis. 



When the rudimentary right gonad develops into a testis, the feathering 

 of the hen changes to the male plumage and the hen begins to crow. It 

 develops male secondary sexual characters, and spermatocytes develop into 

 spermatozoa in this genetic ovary. Thus a complete reversal from a functional 

 female to a functional male is obtained. This result is understandable be- 

 cause the right ovary, which does not normally function, may contain both 

 cortex and medulla. If the left ovary is removed, the right gonad apparently 

 is a labile gonad and can develop in either direction. 



Reversion in the opposite direction is obtained by operative methods in 

 the toad. In the common toad there is a structure at the anterior end of the 

 testis called Bidder's organ. When the testes are removed in an adult toad, 

 Bidder's organ may develop into an ovary with typical large eggs. Histologi- 

 cal examination shows that Bidder's organ is really a small ovary which 

 contains oocytes. This is another case of an adult which is potentially bisexual. 

 Such animals are actually potential hermaphrodites. Indeed hermaphrodites 

 are found in the development of many species of animals. 



This discussion has served to show that just as in the other cases of 

 differentiation considered in earlier chapters the differentiation of the gonads 

 begins with a labile organization. The chromosomes, which act as a stimulus 

 for differentiation, change the original labile organization in one direction or 

 the other. But this control by the chromosomes can be overridden either by 

 the sex hormones — estrone and testosterone — or by temperature, or by simple 

 surgical methods, such as removal of the ovary or the testis, as the case 

 may be. 



With the differentiation of the sex of the individual, the germ cells are 

 destined to differentiate into eggs and spermatozoa in the ovary and testis 

 respectively. Thus the next generation is in the process of formation even 

 before fertilization of the mature egg. In order to complete our story of 

 development, let us consider the events from sex differentiation, which we 

 have just discussed, to ovulation of the egg. This brings us back to the topic 

 with which the book began. 



After sex has been determined by some stimulus released by the chro- 

 mosomes, the primitive germ cells form oogonia and spermatogonia re- 

 spectively. These primitive germ cells undergo both a change in form and a 

 reduction in chromosome number. The reduction in chromosome number 



