122 CARNEGIE INSTITUTION OF WASHINGTON. 



fail or die. For example, in some pigeons the two eggs of the pair or clutch 

 have unequal prospective sex values, and in any complete analysis of sex- 

 ratios in these forms it is necessary to know whether the first or the second 

 of the clutch is more subject to delay and consequent death. Dr. Riddle 

 has, accordingly, carried out and published a study the full purpose of which 

 was to determine if the retention of eggs in the oviduct is a source of embry- 

 onic death; to find out the factor immediately responsible for the death; to 

 measure the amount of death from this cause; and to evaluate the extent to 

 which these eliminations by death may affect sex and genetic ratios obtained 

 in pigeon breeding. In the published results the following conclusions were 

 reached: 



''Approximately 1 egg of each 100 produced by a collection of doves and 

 pigeons has been found to be abnormally retained in the oviduct for periods 

 varying between 5 and 122 hours. The retention or delay of eggs within 

 the oviduct of doves during more than 5 hours results in the death of about 

 one-half of the embryos. About 0.5 to 0.6 per cent of all embryos die as a 

 result of egg-retention in the oviduct. Death usually occurs at or near the 

 0.5-day stage of development. Increased frequency of death occurs in the 

 eggs longest delayed and in eggs provided with thickest shells. Death 

 probably results from an oxygen supply inadequate to the increasing require- 

 ments of the post-laying embryonic stages thus retained in utero. 



"The occasional survival of retained embryos during these early develop- 

 mental stages is a result of specially favorable respiratory conditions, namely, 

 the presence of a thin shell or a delay in the secretion of the normal amount of 

 shell. 



"An egg occasionally remains in the shell gland during two or three times 

 the normal period for shell secretion without obtaining more than the normal 

 amount of shell. More commonly eggs thus retained receive a heavier 

 secretion of shell material. 



"The retention of a 'first' egg of the clutch or pair often or usually results 

 in the suppression of the second egg of the pair. The total result is the 

 death or ehmination from breeding data of approximately one-half of the 

 embryos of both eggs of such pairs. 



"The retention of a 'second' egg of the pair suppresses no later ovulation. 

 This retention of a second egg occurs, however, only once in 200 ovulations, 

 and approximately one-half of these retained embryos survive. The selec- 

 tive elimination of so small a number of second eggs can have but slight 

 effect on the sex-ratio. Even in pure species, where second eggs of the pair 

 are predominantly female-producing eggs, the proportion of females to males 

 is affected from this source by probably less than 0.3 per cent." 



Parthenogenetic Cleavage and the Origin op the Subgerminal Cavity in 



THE Pigeon's Egg. 



The close observation of all infertile eggs encountered in Dr. Riddle's 

 pigeon breeding indicated that such eggs give evidence of the presence of a 

 "subgerminal" cavity — a condition normally associated with an early stage 

 of embryonic development. It had also been learned that the yolks of 

 such incubated infertile eggs absorb very appreciable amounts of water. 

 The interest of Professor G. W. Bartelmez, Department of Anatomy, Uni- 

 versity of Chicago, was enlisted in the appearances connected with some such 

 infertile eggs obtained from "virgin" females, with the result that partheno- 

 genetic cleavage in these eggs was demonstrated. This parthenogenesis in 

 birds is of course a fact of genetic interest, not only because it is found in our 



