CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 325 



which are usually smaller than the one which does, ''polar 

 bodies" whether they lie at the animal pole or not. 



4. Why polar bodies do not develop^ 



Normal polar bodies then are rudimentary eggs which do not 

 develop, though they sometimes divide once or twice. Their 

 failure to develop is usually held to be due to their small size, 

 but even where the polar bodies are quite large, as is sometimes 

 the case in gastropods, polyclads and nematodes, they do not 

 develop. In one case only has the development of a polar body, 

 or rather of two second oocytes, been observed. Francotte ('98) 

 discovered in the polyclad Prostheceraeus that at the first mat- 

 uration division the egg divided into two nearly equal cells; 

 each was then entered by a spemiatozoon and normally fertilized 

 and at the second maturation division each formed a small sec- 

 ond polar body and underwent normal cleavage and developed 

 to the gastrula stage. In a few other instances the entrance of a 

 spermatozoon into a polar body has been reported though some 

 of these cases are not entirely convincing and need verification. 

 Thus Platner ('86) described the entrance of a spermatozoon into 

 a polar body of Arion; he maintained that the polar bodies are 

 formed before the entrance of the sperm, which would make this 

 case similar to that of Prostheceraeus, but the evidence offered 

 is by no means conclusive. Sobotta ('95) calls special attention 

 to the large size of the polar bodies in the mouse and suggests 

 that they may be capable of being fertilized, but offers no evi- 

 dence in favor of this view. Kostanecki ('91) has observed a 

 spermatozoon with its head penetrating the second polar body of 

 Physa, a thing which he regards merely as a 'curiosity.' Lefevi'e 

 ('07) found that the same reagent (HCl) which causes the eggs of 

 Thalassema to develop parthenogenetically also caused the polar 

 bodies to undergo several cleavages. 



The most striking difference between Prostheceraeus and other 

 animals in which giant polar bodies have been reported is to be 



' The substance of this section was summarized in Proceedings National 

 Academy Sciences, 1, pp. 491-496, 1915. 



