PRINCIPAL AXIS OF SYMMETRY IN THE BIRD'S EGG. 325 



of the shell membrane. He did this by killing a hen that was 

 about to lay, ligating the uterus at either end and immersing 

 the whole in a dish of oil. The uterus was then removed and 

 the next day an oil-filled cavity was found in the place of the 

 air space. But why should this space always arise at the blunt 

 end? Coste was able to produce it under any part of the shell 

 by a modification of his procedure. If, instead of removing the 

 uterus under oil he clipped a window in it (thus exposing any 

 given area of the shell) he obtained a typical oil-filled cavity 

 under the window between the shell membranes. He was not 

 able to explain why it normally forms where it does. Valentin 

 (j^SS, p. 30) had previously attributed the formation of the 

 air space to the puckering of the oviducal walls at the junction 

 of "isthmus" and "uterus." He cites Purkinje to this effect 

 but I have not found any mention of the matter in either edition 

 of his paper. The region of the shell where the space forms is 

 obviously more porous, quite possibly for the reason which 

 Valentin gives. I have made some preliminary tests and am 

 inclined to think that the shell cuticle is concerned in this local- 

 ization. 



It would be interesting to know whether there is any connection 

 between the position of the air space and the fact noted by 

 Murray (1826) that the temperature of the albumen at the 

 blunt end is higher than that of the other. 



All birds' eggs which I have examined have the air space at or 

 near the blunt end of the shell. This holds for all pigeons' eggs 

 in which a more pointed end is distinguishable. Some pigeons' 

 eggs, as Coste noted, have equally rounded ends. In such eggs 

 we find the head of the embryo turned away when we hold the 

 air space end to our left. The practical importance of this for 

 the present study is obvious (see p. 341). 



C. Shell Pigment, See discussion on p. 333. 



D. The Chalazce. v. Baer says (1837, p. 18), "ueberhpaut ist 

 nichts im Ei so wechselnd als die Hagelschniire." Previous to 

 1850 this variability gave occasion for much study and contro- 

 versy concerning chalazae. The earliest workers were quite in 

 the dark as to their nature. Aristotle was the first to refer to 

 them and he thought one of them identical with the stalk of the 



