CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 323 



normal development result, proves that there is no essential dif- 

 ference in the two poles of the maturation spindle. Also the 

 return of the egg nucleus to the animal pole after centrifuging 

 and after the formation of a polar body at some other pole of 

 the egg is probably due to attachments which connect the 

 nucleus to the animal pole. 



• Because of the elongation of the spindle in these centrifuged 

 eggs, the polar body which is formed may be extraordinarily 

 large. If centrifuging occurs during the first maturation divi- 

 sion, the first polar body is the giant one; if during the second 

 maturation, it is the second polar body ; if during both maturation 

 divisions, both polar bodies are abnormally large. In my ex- 

 periments the giant polar body is usually the second one, since 

 many more eggs were centrifuged during the second maturation 

 division than during the first. The cell constrictions shown in 

 figures 23 to 25 lie opposite the equator of the second matura- 

 tion spindle and indicate that giant polar bodies are about to be 

 formed. These giant polar bodies may be formed at the animal 

 pole or at any other point on the surface of the egg ; they may con- 

 tain the oil of the light zone, the cytoplasm of the middle zone 

 or the yolk of the heavy zone or they may contain samples of all 

 these substances, depending upon whether they are formed at the 

 centripetal or the centrifugal pole and also upon their size (figs. 

 32 to 34, 43 to 55, etc.). 



Normal first polar bodies usually divide by mitosis, second 

 polar bodies rarely do, but I have never seen a case in which a 

 giant polar body divides, and this in spite of the fact that the 

 polar body may contain samples of all the egg substances and 

 may be as large as, or even larger than, the rest of the egg. In 

 most cases the chromosomes of the polar bodies never form a 

 resting nucleus nor even chromosomal vesicles but remain as dis- 

 tinct chromosomes up to the latest stage studied (figs. 53, 54, 

 56). In a few instances these chromosomes form chromosomal 

 vesicles or even a resting nucleus (figs. 49, 50, 51). But in only 

 one instance (fig. 57) have I observed an indication of an ap- 

 proaching division of a giant polar body and this case is a very 

 doubtful one. 



