ON THE OVARIES IN CERTAIN BRITISH BIRDS. 63 



0. On the Presence of two Ovaries in certiiin British Birds, 

 more especially the Falconida3. By T. E. Gunn, F.L.S.* 



[Received October 23, 1911 : Read November 7, 1911.] 

 (Plates ll.-V.t) 



In all vertebraterl animals the reproductive organs or gonads 

 are paired symmetrical glands lying to the inner side of the 

 Wolthan body and in fi'ont of the kidney in the dorsal aspect of 

 the body -cavity. 



In the early stages of development these paired glands are 

 found in every vertebrate from Fishes upwards. Morphologically 

 indifferent at first, they become differentiated later on into the 

 essential male (testis) or female (ovary) glands. 



In exceptional cases a further change takes place leading to the 

 suppression of one of the female gonads with its appendage, e. g., 

 certain Elasmobranch fishes, and — it is said — the whole class 

 Aves. The suppression is supposed to be connected with the 

 large size of the eggs developed by the individual, e. g., Scyllium 

 in fishes and Uria in liirds. 



In the embryo chick (of the fowl) the ridge of germinal 

 epitheliinn is sufficiently differentiated by the fifth day to determine 

 whether the primordial germ-cells of which it is largely composed 

 are destined to furnish the individual with the permanent ova 

 characteristic of the hen, or the spermatozoa of the cock bird : in 

 other words, the ovaries are distinct from the testes at this 

 date. 



With regard to the male organs, their development pursues a 

 normal course for the remainder of embryonic life, and the male 

 chick steps out of the shell with two synunetrically placed and 

 permanent functional testes. 



With the female organs a further negative change takes place. 

 The development of the ovaiy on the right side comes to a 

 standstill and the organ atrophies, leaving little or no trace of its 

 former presence. The oviduct dwindles with the ovary and is 

 finally lost almost entirely ; vestiges may be found moi'e especially 

 at the cloacal end of the tube, but it is never complete from end 

 to end as a functional duct. 



Exactly when this degeneration begins, and at what period of 

 embryonic life the chick will be found with only the single left 

 ovary, are apparently uncertain ; but it is an accepted fact that 

 the newly hatched female chick has but one ovary — that on the 

 left side — and this one gland has to serve for the purposes of 

 reproduction during the whole of her ovum-bearing existence. 



Morphologists and ornithologists alike are agreed that adult 

 female birds have but one functional ovary. Very few authors 



* Communicated by F. Menteitii Ogilvie, F.Z.S. 

 t For explauatioii of the Plates see p. 79. 



