DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 5 



around it. It varies in width from about 15 mm. to 35 mm., being 

 narrowest at its first appearance. . Sometimes its borders are quite 

 sharp and even (fig. ia) ; in other cases they are very irregular. If 

 the embryo dies the chalky band is likely to become spotted with 

 dark areas. 



The shell and shell membrane of the egg of the Madagascar croco- 

 dile are essentially the same as those just described, except that the 

 shell is sometimes pierced by small pores that pass entirely through 

 it. The same chalky band surrounds the median zone of the egg 

 (18). 



The white of the egg is chiefly remarkable for its unusual density, 

 being so stiff that the entire egg may be emptied from the shell into 

 the hand and passed from one hand to the other without danger of 

 rupturing either the mass of albumen or the enclosed yolk. The 

 albumen, especially in the immediate neighborhood of the yolk, seems 

 to consist of a number of very thin concentric layers. It varies in 

 color, in different eggs, from a pale yellowish white, its usual color, 

 to a very decided green. 



As might be expected, no chalazas are present. 



The yolk is a spherical mass, of a pale yellow color, lying in the 

 center of the white. Its diameter is so great that it lies very close to 

 the shell around the lesser circumference of the egg, so that it is 

 there covered by only a thin layer of white, and care must be taken 

 in removing the shell from this region in order not to rupture the 

 yolk. The yolk substance is quite fluid and is contained in a rather 

 delicate vitelline membrane. 



The albumen and yolk of the crocodile's egg, as described by 

 Voeltzkow, differ from those of the alligator only in the color of the 

 albumen, which in the crocodile is normally light green (18). 



As pointed out by Clarke, the position of the embryo upon the yolk 

 is subject to some variation. During the earliest stages it may occur 

 at the pole of the yolk nearest the side of the egg ; later it may gener- 

 ally be found toward the end of the egg; and still later it shifts its 

 position to the side of the egg. It is probable, as Clarke says, that 

 the position at the end of the egg secures better protection by the 

 greater amount of white, at that point, between the yolk and the 

 shell ; while the later removal to the side of the egg, when the vascu- 

 lar area and the allantois begin to function, secures a better aeration 

 of the blood of the embryo. 



Around the embryo, during the stages that precede the formation 

 of the vascular area, is seen an irregular area of a lighter color and a 

 mottled appearance. This area is bounded by a distinct, narrow, 



