218 THE CONTINUITY OF THE GERM-PLASM AS THE 







nature, in the possession of a peculiar cell-body, frequently con- 

 taining food-yolk, and of the nucleus which is characteristic of all 

 animal eggs during the process of growth. Hence we cannot 

 doubt the presence of a specific ovogenetic nncleoplasm, and must 

 therefore also believe that it is ultimately removed in the polar 

 bodies. 



In other Coelenterata, in worms, echinoderms, and in molluscs 

 polar bodies have been described, as well as in certain Crustacea, 

 viz. in Balanus by Hoekand in Cetochilus septentrional e by Grobben. 

 The latter instance appears to be quite trustworthy, but there is 

 some doubt as to the former and also as regards Moina (a Daphnid), 

 in which Grobben found a body, which he considered to be a polar 

 body, on the upper pole of an egg which was just entering upon 

 segmentation. In insects polar bodies have not been described up 

 to the present time \ and only in a few cases in Vertebrata, as in 

 Petromyzon by Kupffer and Benecke. 



It must be left to the future to decide whether the expulsion of 

 polar bodies occurs in those large groups of animals in which they 

 have not been hitherto discovered. The fact, however, that they 

 have not been so discovered cannot be urged as an objection to my 

 theory, for we do not know a priori whether the removal of the ovo- 

 genetic nucleoplasm has not been effected in the course of phylogeny 

 in some other and less conspicuous manner. The cell-body of the 

 polar globules is so minute in many eggs that it was a long time 

 before the cellular nature of these structures was recognized 2 ; and 

 it is possible that their minute size may point to the fact that 

 a phyletic process of reduction has taken place, to the end that the 

 egg may be deprived of as little material as possible. It is at all 

 events proved that in all Metazoan groups the nucleus undergoes 

 changes during the maturation of the egg, which are entirely similar 

 to those which lead to the formation of polar bodies in those eggs 

 which possess them. In the former instances it is possible that 

 nature has taken a shortened route to gain the same end. 



It would be an important objection if it could be shown that no 



1 They have now been observed in many species, so that their general occurrence 

 in insects is tolerably certain. Compare bibliography given in Weismann and 

 Ischikawa, ' Weitere Untersuchungen zum Zahlengesetz der Richtungskb'rper,' 

 'Zoolog. Jahrbiicher,' vol. iii. 1888, p. 593. A. W., 1888. 



* Van Beneden, even in his last work, considers these bodies to have'only the value 

 of nuclei; 1. c.. p. 394. 



