30 THE DEVELOPMENT OF THE CHICK 



implies a certain periodicity in the deposition of the yolk that 

 is not understood. 1 



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, with 

 the protoplasmic reticulum that forms the walls of the yolk- 

 vacuoles. The protoplasmic reticulum gradually disappears as 

 the vacuoles are completely converted into yolk-granules. In 

 the last stages of growth a peripheral layer of white yolk is formed 

 around the entire yolk-mass, presumably by the peripheral proto- 

 plasmic layer. The germinal disc increases in extent and thick- 

 ness; but whether this takes place by inflowing of the peripheral 

 protoplasm, or growth of the original disc, is not known. It is 

 certain that the peripheral protoplasm disappears over most of 

 the yolk when the external layer of white yolk is formed. An 

 inflow of the peripheral protoplasm into the disc appears very 

 probable by analogy with the bony fishes where this process can 

 be studied with great ease. The alternative assumption would 

 be the complete utilization of the peripheral protoplasm in form- 

 ing the superficial layer of white yolk. 



The method of formation of the neck of the latebra and the 

 so-called nucleus of Pander, or peripheral expansion of the neck, 

 follows more or less directly from the preceding account: As the 

 circumference of the ovum enlarges, the germinal disc is carried 

 out and leaves behind it a trail in which yellow yolk is not formed. 

 When the ovum is fully grown, the exact boundaries between the 

 protoplasmic germinal disc and the yolk are not determinable. 

 The disc itself is charged with small yolk-granules which grade 

 off very gradually into the white yolk lying around and beneath 

 the disc. 



The mode of nutrition of the ovum and the formation of the 

 vitelline membrane remain to be considered. The nutrition is 

 conveyed from the highly vascular' theca folliculi by way of the 

 follicular cells, or membrana granulosa, to the ovum. The nutri- 

 ment enters by diffusion; at no stage is there any evidence of 



1 Since the above was written, observations and experiments of Dr. Oscar 

 Riddle have demonstrated that the periodicity is daily, and is correlated 

 with the daily physiological rhythm of vitality of the hen. Acknowledgment 

 is due Dr. Riddle for permission to make this statement in advance of his 

 own publication. 



