THE EGG 29 



While the nucleus is still near the center of the egg a very 

 dense deposit of extremely fine granules is formed around it, and 

 gradually extends out towards the periphery of the cell, ]>ut 

 does not involve the peripheral layer of protoplasm. This central 

 aggregation of yolk-granules represents the primordium of the 

 latebra or central mass of the white yolk. 



The ovum grows very slowly up to a diameter of about 6 

 millimeters, and all of the yolk found during this period belongs 

 to the category of white yolk. Certain of these ova, but only a 

 few at any one time, then suddenly begin to grow at an enor- 

 mously increased rate, adding about 4 mm. to their diameter 

 every twenty-four hours until the full size of about 40 mm. in 

 diameter is attained. It is during this period that the concentric 

 layers of yellow and white yolk are laid down in the periphery. 



Riddle has studied this period by the ingenious method of 

 feeding the stain Sudan III, which has an especial affinity for fat, 

 to laying hens at definite time intervals. The stain attaches itself 

 to fatty acids of the food which are taken up unchanged by the 

 egg. The consequence is that during any period of Sudan III 

 feeding a red stained layer of yolk is formed; so that it is possible 

 by regulating the dose and interrupting the feedings to obtain 

 ova with alternate bands of stained and unstained yolk. In this 

 way he was able to show that a layer of yellow and of white yolk 

 about 2 mm. in combined thickness on the average is laid down 

 each twenty-four hours. 



In a previous study the same author had shown that there is 

 a daily rhythm of nutrition, associated with high and low blood 

 pressure, which is responsible for the formation of the alternate 

 fault-bars and fundamental bars of birds' feathers. It is this same 

 daily rhythm that determines the concentric stratification of the 

 yolk, yellow yolk being formed during the longer period of high 

 blood pressure, and white yolk during the briefer nocturnal period 

 of low pressure. 



''The layer of white yolk of the hen's egg is then a growth- 

 mark left at the ever changing boundary of the ovum ; it represents 

 the result of yolk formation under sub-optimal conditions." 

 (Riddle.) 



The germinal vesicle lies in a thickening of the peripheral 

 layer of protoplasm known as the germinal disc, which is con- 

 tinuous, like the remainder of the peripheral protoplasm, in early 



