300 



EMBRYOLOGY 



MESONEPHRIC 

 TUBULE 



Fig. 192. The early origin of 

 the germinal epithelium from the 

 splanchnic mesoderm of an early 

 chick embryo. 



GERMINAL EPITHELIUM 



gonads arise in part from a thickening of the splanchnic mesoderm and 

 from intermediate mesoderm called the germinal epithelium (Fig. 192). 

 From this germinal epithelium a saclike structure develops which has two 

 important groups of cells — an inner layer of cells, the medulla, and an outer 

 layer of cells, called the cortex (Fig. 193) . Both male and female possess the 

 medulla and the cortex, and the differentiation of an ovary occurs by a 

 proliferation of the cortex. The ovary is characterized by a differentiation of 

 the cortex, which forms the actual tissue of the ovary itself, while the medulla 

 undergoes a regression. In the case of the formation of the testis, it is the 

 medulla which increases and differentiates, while the cortex usually dis- 

 appears. 



There is one other fact that must be considered in any analysis of this 

 change. It is a peculiarity of the chick embryo that only the left ovary develops 

 into a functional ovary (Fig. 193). The primitive right gonad in the female 

 chick does not differentiate but remains in an embryonic condition. The 

 rudimentary right gonad may retain both a medulla and a cortex. This 

 asymmetry of sex differentiation is an important fact to remember in the 

 interpretation of experiments done on the reversal of sex in birds. 



Under normal conditions the chromosomes direct the sex differentiation 

 of this primitive gonad, which contains both cortex and medulla. However, 

 the differentiation is a labile one, subject to change, since if either estrone or 

 testosterone is injected into the egg it is possible to reverse the normal 

 chromosome control and to some extent reverse the sex of the individual. 

 The injection of estrone into a 48-hour chick egg brings about drastic changes 

 in the development of the gonads. For this experiment some sort of marker 

 is needed to indicate which embryos would normally become males. We will 

 call these embryos genetic males, and they are recognized, when they hatch, 



