Patten : Dimorphism in Eggs^ etc. 277 



I was much interested with the aborted condition of the 

 yelk, and it further occurred to me to obtain, if possible, evi- 

 dence regarding fertility. This I was enabled to do after much 

 delicate manipulation. For the eggs were fresh, and, though 

 the vitelline membrane gave way as the contents were being 

 extruded, still, after a careful search which occupied several 

 hours, I managed to secure the germinal disc, and to isolate 

 it from the yelk in each egg. In the case of the three spotted 

 shells, the germinal discs, which measured 4 mm. in diameter, 

 showed evidence of fertility, for development had proceeded as 

 far as the early indication of the embryonic shield, the primitive 

 streak being barely visible. In the disc of the abnormal egg, there 

 were no traces of developmental activity having taken place, and , 

 to the best of my belief, fertilization had not ensued. Here, then, 

 is an interesting association, viewed in its physiological aspect, 

 between variation in shell structure, absence of the pigmental 

 deposit from the villous membrane of the parent's uterus, an 

 abnormally small sized yelk-ball, which was unusually light in 

 colour, and non-fertility. That there should be any necessary 

 association between arrested activity of the secretion of the pig- 

 mental deposit, which is not laid down until the shell is formed, 

 and non-fertilization of the ovum itself, is not at all evident, 

 and further investigation into the matter would be interesting. 



I may conclude with a brief reference to the pigment spots 

 on the three other eggs of the clutch. In addition to a general 

 distribution of small circular spots, great irregularly-shaped 

 blotches are to be seen. With the exception of the lower spot 

 on the face of egg No. 3, they are reddish-brown in colour ; 

 indeed, these eggs, especially Nos. i and 2, might almost pass 

 for that type of Ring-Ouzel's egg, which one occasionally meets 

 with, displaying a clear bluish ground-colour with discreet 

 brownish blotches. The difference in the black pigment spots 

 seen on some Thrushes' eggs and the rusty reddish-brown on 

 others, depends upon the thickness with which they are 

 deposited. The pigment is naturally dark reddish-brown, and 

 when laid on thinly, appears as such ; a thick coating appears 

 almost black.* In this clutch, therefore, the pigment was 

 evidently sluggishly secreted and deposited, until after attempts 

 were made to spot three eggs, the secretive power of the gland 

 finally ceased, leaving one egg altogether free from spots. 



* Just as in the case of the super-posing of hundreds of coloured 

 blood-corpuscles, straw-yellow in shade, gives one the idea of rich red 

 blood. 



igog Aug. i. 



