54 M. R. CURTIS AND RAYMOND PEARL 



amount of free j^olk. A little yolk was oozing through a small 

 hole at the mid ventral angle of this pocket. There were a few 

 small lumps of yolk among the viscera. When the pocket 

 was opened and the free yolk wiped out the ovary was found to 

 be in normal laying condition. It contained a series of four 

 growing yolks, the largest mature, and seven resorbing follicles. 

 The oviduct was in laying condition. 



At the time of autopsy, then, this bird was ovulating into the 

 ovarian pocket and absorbing the yolks. The rate of absorption 

 was evidently not equal to the rate of yolk formation. In 

 consequence the pocket was stretched to its capacity and had 

 given way at one point, allowing a little yolk to escape into the 

 body cavity. 



The egg record of this bird shows neither eggs nor nesting 

 records until December 22. From then on until the end of the 

 year the nesting records follow a rhythm similar to a very slow 

 laying rhythm. There is one egg recorded on February 2, a 

 normal clutch of three on March 13, 15 and 17 and a single egg 

 again on May 26. While it is possible that records of all these 

 eggs are mistakes due to causes discussed in reference to the 

 egg record of No. 141, it is probable that at least the record of 

 the normal clutch in March is authentic, as the errors considered 

 would hardly have resulted in this sort of a record. It seems 

 probable that the pocket was neither congenital nor formed 

 complete at once but that during its growth it first hindered and 

 finally prevented the entrance of yolks into the oviduct. 



In any case, the immediate cause of the partial sterility 

 exhibited is somatic, in the sense that it is not directly con- 

 nected with or related to the genetic constitution of the bird in 

 respect to fecundity. 



Bird No. 431 represents a case of somatic steri ty of a different 

 sort from those so far considered, in that the difficulty was not 

 primarily with the genital organs. This bird had a crippled 

 back which interfered with the normal use of her legs; she was 

 in poor flesh; she had never been in a trap-nest either to lay or 

 nest. At autopsy the sex organs were in strictly non-laying 

 condition. There was no visible obstruction between the ovary 



