148 



GENERAL EMBRYOLOGY 



and share in the development of a new organism. The other 

 three (or two), the polar bodies, after a brief time degenerate 

 and disappear. 



The view that the ovum and the polar bodies are equivalent 

 cells morphologically, and that the latter are in reality to be 

 looked upon as degenerate egg cells (Mark) is now familiar. 

 Their identity in nuclear structure and history is of course a 

 decisive similarity. 



FIG. 79. Variations in the size of polar bodies. A, B. Sections through 

 segmenting ovum of the Gasteropod, Limax maximus, showing polar bodies of 

 very different sizes. After Meisenheimer. C-F. Ascaris megalocephala. C, D, 

 E, after Sala, show influence of low temperature; F, after Boveri. C. Large 

 first polar body which has divided. D. Very large first polar body. E. Equal 

 division of egg during "first polar division." F. Equal division of egg during 

 "second polar division." /, //. First and second polar bodies; 9, egg pro- 

 nucleus; d\ sperm pronucleus. 



The actual size of the polar bodies varies enormously. Those 

 of the Echinoderms are among the smallest, some being only 

 5-8 micro, (1/5000-1/3000 inch) in diameter; in Amphioxus 

 they are about 7 micro, or about one-fourteenth the diameter 

 of the egg, while in the mouse they are relatively much larger 

 13 micro, or one-fifth the diameter of the egg. An interesting 

 series of forms can be arranged bridging over the size differences, 

 and to some extent also the physiological differences between 

 the ovum and the polar bodies. In a few Annelids, some 



