BIRDS' EGGS 247 



they are formed, about the seventh day, it is possible to dis- 

 tinguish the sexes, for the number of primordial sex-cells 

 remaining in the germinal epithelium after the formation 

 of the cords of first proliferation is small in the male, and not 

 greatly diminished in the female. 



The strands of first proliferation are ephemeral as in 

 mammals. They disappear a few days after hatching, and 

 a chicken a fortnight old shows not a trace of them. Their 

 place is taken by a second proliferation of germinal epithe- 

 lium — the cortical strands — which form the cortical zone 

 of the ovary, and it is from their elements that the definitive 

 ova arise. Firket (1914) points out an interesting contrast 

 between the medullary strands and the cortical strands of 

 the ovary. The former illustrate retrogressive changes, for 

 the immature ova they produce, though numerous at the 

 end of incubation, never come to anything, and never get 

 a follicular envelope. The cortical strands, on the other 

 hand, show progressive changes ; they form the ova which 

 become invested in a follicular envelope. 



Beginning with the 8th day, but especially during the 

 9th, loth, and nth days of development, there is a rapid 

 increase in the number of primordial sex-cells or oogonia 

 in the germinal epithelium of the female embryo. They 

 form groups, which are the results of successive divisions, 

 and give rise to lobulations or buds on the deep surface of 

 the germinal epithelium. These buds are composed chiefly 

 of oogonia, but they include cells of peritoneal origin. As 

 the buds increase in size and become the cortical cords or 

 the cords of second proliferation, the oogonia become 

 definitive ova, while the peritoneal cells of the germinal 

 epithelium, present in the cords, develop the follicular 

 epithelium, which forms an envelope round the growing egg. 



Looking backwards we may distinguish in this com- 

 plicated history the following stages in the development of 

 the bird's ovary : — 



(i) Primordial sex-cells (" primary gonocytes ") appear 

 very early, and do not share in body-making. They migrate 

 in the embryonic tissue, and most of them probably 



