MUSEUM OF COMPARATIVE ZOOLOGY. 187 



is of short duration. At a subsequent stage it will be seen that this 

 monaxial Iiaploipoliir form is followed by a monaxial diploipolsir. Po- 

 tentially, I believe, this diplopolar condition already exists. In the 

 present state of our ignorance as to the nature and residence of the 

 forces which control the phenomena connected with maturation, it is not 

 possible to find direct evidence of such a state at this early stage. In 

 the eggs of many other animals, the diplopolar condition is manifest 

 much earlier.* 



The protoplasm of the yolk not embraced by the spindle and amphi- 

 aster is closely crowded with highly refractive deutoplasmic granules, 

 which do not exhibit any system of arrangement, save that they are uni- 

 formly distributed. These granules are irregular in form after the 

 action of acid, and they vary considerably in size. In any optical sec- 

 tion, the peripheral portions of the yolk appear more translucent than 

 the central portions, not necessarily because of less abundance or less 

 density of the deutoplasmic elements, but simply from the diminished 

 amount of yolk substance which the light has to traverse to reach the 

 observer's eye. 



It would not be true, however, to say that the granulations extend 

 quite up to the surface of the egg. There is a very thin, almost 

 imperceptible layer of substance, free from deutoplasmic elements, which 

 forms the outer envelope of the yolk. This is in no way to be con- 

 sidered as a vitelline membrane ; however sharp the external boundary 

 may be, the internal portion merges so gradually into the yolk substance 

 as to afford not the slightest ground for assuming that it is a distinct 

 membrane. It is hardly necessary to add, that there is no evidence of 

 a double contour, and that all attempts to separate as a distinct structure 

 this outer condensed portion of the yolk are quite futile. 



As will be seen from Fig. 39, the uniformity in the distribution of 

 the yolk granules is interrupted at irregular intervals (a). Spaces im- 

 mediately contiguous to the surface of the yolk appear to be quite 

 destitute of granules, and the corresponding portions of the surface are 

 often raised into transparent, boss-like protuberances. These spaces — 

 a dozen or more in number — are irregularly distributed over the whole 

 yolk. They at once become conspicuous when the egg is placed in 



* I am unable to say whether the axis of the yolk corresponding with the axis of 

 the maturation spindle is identical with that which becomes differentiated at the 

 formation of the polar globules. If so, then the migration of the spindle is only a 

 motion of translation along this axis ; but, on the other hand, if the spindle at any 

 time assumes a position oblique to the radius which passes through its centre, it is 

 probable that the axes are not the same. 



