EMBRYOLOGY OF CRYPTOBRANCHUS 149 



into the central cavity; in a broad sense, the entire three-layered 

 structure may be called the follicle. 



The zona radiata is formed from the peripheral substance of 

 the egg proper; at the time of the rupture of the germinal vesicle 

 it becomes transformed into a simple cell wall, in organic connec- 

 tion with the egg. 



The zona pellucida is formed as a secretory product of the fol- 

 hcular layer proper; it persists unchanged as the ' vitelline mem- 

 brane' of the embryo. 



The earliest observed phenomena which may perhaps indicate 

 polarity occur in the ovarian eggs of young females of a body 

 length of 26 to 30 cm., as a shifting of the region of most abundant 

 vitelline bodies from the future vegetal to the future animal hemi- 

 sphere. In the ovarian eggs of young females of a body length 

 of 35 cm there is a concentration of nucleoli on the side of the germ- 

 inal vesicle toward the future animal pole; this may perhaps 

 afford a second indication of polarity. 



Yolk-formation begins in the most advanced ovocytes of young 

 females with a body length of 35 cm. ; the yolk is first laid down in 

 concentric zones. With respect to the position of the germinal 

 vesicle, the distribution of cytoplasm, and the size of the yolk 

 particles in the different zones, the egg exhibits radial symmetry 

 until after it is nearly or quite filled with yolk. 



About the time when the egg becomes completely filled with 

 yolk, the germinal vesicle migrates from its central position toward 

 a point on the surface which is thus defined as the animal pole. 

 Coincident with the migration of the germinal vesicle, axial differ- 

 entiation of the cytoplasmic and yolk contents of the egg lead to 

 the formation of a germinal disc in the region of the animal pole. 



In general the animal pole of the egg lies within the stalk of the 

 follicle and toward the periphery of the ovary. 



In the late ovarian egg a structure called the yolk cup is inter- 

 preted as the physiological equivalent of the concentric layers of 

 dense fine yolk found in the eggs of birds and various other verte- 

 brates. 



Shortly before maturation the germinal disc is temporarily 

 differentiated into two layers: a thin peripheral layer of yolk- 



