PRINCIPAL AXIS OF SYMMETRY IN THE BIRD'S EGG. 333 



inverted in the "uterus" of this species. In such cases it is prob- 

 able that the egg is rotated late in its uterine history. 



Shell Pigment. The unequal distribution of the pigment can 

 now be discussed. While the method of distribution has been 

 worked out satisfactorily, the origin of the pigment still remains 

 obscure. As this field is terra incognita to most embryologists 

 and ornithologists it is worth a digression. Wicke in 1858 laid 

 the foundation for the understanding of the matter. He dis- 

 covered that the pigment spots which are various shades of red 

 or green, are closely related to or identical with the blood pig- 

 ments bilirubin and biliverdin. Leuckart (1853) suggested that 

 they might be derived from blood extravasated from the hyper- 

 semic oviducal vessels. Kiitter (1878) made the first direct 

 observations on the subject. He found in the "uterus" of a 

 Falco tinnunculus an unpigmented egg, although the eggs of this 

 species are normally deeply pigmented. Scattered over the 

 surface of the oviduct were small red-brown masses presumably 

 passing down, for in other oviducts he found similar masses being 

 incorporated into the outer layers of the shell. He suggested 

 that these pigment masses were secreted in the upper part of the 

 duct. In 1885 Taschenberg confirmed these observations and 

 showed that there are no pigment glands in the oviduct, confirming 

 Nasse's (1862) previous observations. He concluded therefore 

 that the pigment must be derived from the follicle at the time of 

 ovulation. He pointed out for the first time the significance of 

 the greater massing of pigment at the blunt end with reference 

 to the passage of the pigment down the duct and the normal 

 position of the egg in the uterus. It is obvious therefore that the 

 unequal distribution of the pigment clearly marks the principal 

 egg axis and shows that in all cases where there is unequally 

 distributed pigment, the egg is oriented as it is in the 'hen and 

 pigeon. In 1893 Wickman published a monograph on the pig- 

 ment of the shell dealing with numerous species, confirming 

 Kiitter and independently reaching conclusions similar to those 

 of Taschenberg. He did not however obtain any conclusive 

 evid:nce for the origin of the blood pigment from the ruptured 

 follicle. This is an opportunity for an exceedingly interesting 

 study involving also the nature of the oxidations of the blood 

 pigments. 



