CELL-ORGANS OF SEA-URCHIN EGG 565 



slightly granular (fig. 1). Thus, the cytoplasm of a ripe but 

 immature egg is difTerentiated in three layers, concentrically 

 arranged around the nucleus. 



The maturation of the egg leads to extensive changes both in 

 the nucleus and in the cytoplasm. The most striking phenom- 

 enon is the change in the relation of the nucleus and the cyto- 

 plasm, this change occurring quite abruptly (fig, 2). 



Hertwig, in his studies, on 'Kernplasmarelation,' finds in the 

 immature egg conditions of depression, similar to conditions of 

 senescence in Actinospherium and Infusoria, in which cases the 

 nucleus also acquires large dimensions. According to him, the 

 reduction of the nucleus during maturation enables the further 

 development of the egg cell. Minot and Conklin on the' con- 

 trary attribute to the increase of nucleus a meaning of rejuve- 

 nescence and believe that senescence depends upon the increase 

 of the cytoplasm and upon its differentiation. 



These differences of opinion regarding the bearing *of the 

 Kernplasmarelation may be accounted for by studies of cellular 

 processes, without due consideration of their mutual relation. 

 Decrease or increase of the coefficient of the Kernplasmarelation 

 may depend upon different causes and can have different mean- 

 ings. 



Conklin and Minot rightly consider the fall of the coefficient 

 of the Kernplasmarelation as an indication of sensecence, but 

 they refer to a fall of the coefficient, due to a collection of differ- 

 entiation products in the cytoplasm. The fall of the coefficient 

 during maturation of the egg is of different origin, therefore has 

 a different meaning. Have the changes, which are observed 

 during maturation, the character of differentiation processes? 

 On the contrary, the structure of a mature egg becomes simpler. 

 The morphological structures of the cell which characterize an 

 immature egg, such as the nucleolus and the filaments in the 

 nucleus, are no longer discernible in the mature egg. Even the 

 three different layers of cytoplasm are reduced to two: a very 

 thin peripheral layer, traversed by numerous parallel striae 

 and the greater central mass of the cytoplasm. The egg now 

 contains a very small nucleus. The greater part of the contents 



