480 J. BRONTE GATENBY 



The peripheral clear area (cl), the cone of protoplasm 

 (acp), and the latebra are generally described as containing 

 white yolk-spheres, the rest of the egg mainly yellow 

 yolk-spheres. 



The clear thin layers of concentric stratification (cr) have 

 been said to contain white yolk-spheres, though this has not 

 been settled satisfactorily. Riddle (5), however, believes that 

 the concentric layer does contain white yolk, and is a growth- 

 mark. 



Semon's description of the yolk of the egg of Ornitho- 

 rhynchus does not include any mention of these concentric 

 layers of stratification within the egg, but in his figures he 

 shows eggs of Echidna and Ornithorhynchus which contain 

 one (Tafel VIII, fig. 23), two (fig. 25 and fig. 19), and three 

 (fig. 20) layers, as depicted in Text-fig. 1 (cr) of this paper. 



It is possible that the egg of Ornithorhjaichus might contain 

 these concentric lines of growth, if such they be. The varving 

 number of lines are probably significant of the different periods 

 or epochs of the year during which the eggs grew most ; that 

 with two rings possibly grew in two sudden well-marked 

 periods, and so on. This opinion is supported by Riddle's work 

 on feeding Sudan III to laying fowls (5). 



4. General Account of the Formation of Egg- 

 membranes IN Sauropsida and Mammalia. 



In a recent paper (8) Miss Alice Thing has studied the forma- 

 tion of the zona pellucida in various turtle eggs. When the 

 young oocyte of the turtle has reached a size two or three times 

 that of the oogonium, it becomes surrounded by a flattened 

 epithelium which persists as one layer throughout the course of 

 development of the egg. With the gradual growth of the 

 oocyte, the epithelial cells take on a definite prismatic shape 

 and increase in height in the axis perpendicular to the surface 

 of the egg. Occasional mitoses prove, that to accommodate 

 the increasing volume of the egg, the epithelium extends itself 

 by division of its constituent cells ; in very large eggs numerous 

 mitoses occur. The epithelial cells forming the follicle are 



