MATURATION 149 



Turbellaria, and most Molluscs (Fig. 79) the polar bodies are 

 very large, sometimes even one-fourth the diameter of the 

 egg itself. In occasional instances, abnormal " giant" polar 

 bodies are formed, which really approach the ovum in size 

 (e.g., Amphioxus, and the Turbellarian, Prosthecerceus) . Some 

 of these large polar bodies form a definite membrane, like the 

 vitelline membrane of the ovum; and in rare instances they are 

 actually fertilizable, although their development never proceeds 

 beyond an incomplete cleavage. The last step in the transition 

 from polar body to egg cell is represented by an interesting 

 condition occasionally found in Ascaris and some other forms, 

 (e.g., mouse) where an abnormally placed polar spindle may 

 result in the division of the oocyte into two equal cells, one of 

 which should be called a polar body (Fig. 79, E, F). The last 

 step in the opposite direction the dissimilarity between egg 

 and polar body reaches a climax in some of the Insects where 

 polar bodies are not really formed as such; the oocyte nucleus 

 divides as in polar body formation and daughter nuclei are 

 formed but these remain in the periphery of the egg cell, for 

 no cytoplasmic division whatever is accomplished. The 

 nuclear phase is the only part of the maturation division 

 remaining. The "polar nuclei" formed in this way degenerate 

 without sharing in development, just as if they had been cast 

 out of the cell. 



We have already suggested that this dissimilarity in size 

 between the polar bodies and the ovum is in the nature of an 

 adaptation such that maturation of the egg nucleus may take 

 place without reducing the amount of cytoplasm and food 

 materials which are to form the chief portion of the 

 material basis of the new organism. In accomplishing this, 

 three of every four 'potential egg cells are totally deprived of 

 these substances and lose all possibility of developing. In 

 some few instances, the polar bodies may for a time remain 

 alive and during the early cleavage show some signs of activity, 

 such as the performance of amoeboid movement, "spinning 

 activity," etc. (Andrews). 



The location of the polar bodies within or without the vitel- 



