120 BERTRAM G. SMITH 



defined by a point on the surface, toward which the germinal 

 vesicle is moving. The cytoplasm is now everywhere thickly 

 interspersed with yolk granules; these granules are in general 

 coarse throughout the central portion of the egg, finer and 

 more densely packed at the periphery. Axial differentiation in 

 the arrangement of yolk particles is now for the first time evi- 

 dent in a slight thickening of the peripheral layer of fine yolk 

 particles in the region of the animal pole. This region is also 

 somewhat richer in cytoplasm than the remainder of the egg. 

 There is thus present the beginning of a germinal disc or blasto- 

 disc, which in later stages becomes visible in surface views as 

 the germinal area. 



In the vegetal hemisphere a region of particularly fine and 

 dense yolk, crescent-shaped in meridional section, lies mid-way 

 between the center of the egg and the periphery. This region I 

 shall call the 'yolk cup.' Its appearance suggests that it may be 

 a part of a once continuous zone completely enclosing the germinal 

 vesicle, and that, in the animal hemisphere, this zone has been 

 interrupted in consequence of the migration of the germinal 

 vesicle toward the surface. Probably the yolk cup is the physio- 

 logical equivalent of the concentric layers of dense fine yolk found 

 in the egg of the hen and various other vertebrates. Riddle ('11) 

 has shown that the alternate layers of yellow and white yolk in the 

 hen's egg are due to a daily rhythm in nutrition; he has advanced 

 the same principle in explanation of the concentric layers of 

 yolk in the eggs of certain cyclostomes, selachians and reptiles. 

 In Cryptobranchus, from comparison with ovarian eggs taken in 

 the autumn after the close of the spawning season, it is evident 

 that in the stage under consideration the yolk cup marks the limits 

 of growth during the preceding winter; hence it seems very prob- 

 able that the yolk cup is the result of a seasonal variation in 

 nutrition, and represents a layer added during the winter months. 



The nucleoli are still found mainly at the periphery of the ger- 

 minal vesicle, but with no constant tendency toward concentra- 

 tion in an axial position. 



It has been noted that the animal pole, as defined both by the 

 center of the germinal disc and the point on the surface toward 



