222 CONTINUTIY OF THE GERM-PLASM AS THE [IV. 



find that polar bodies have not yet been discovered in sponges ', 

 but this negative evidence is no proof that they are really absent. 

 In all probability, no one has ever seriously endeavoured to find 

 them, and there are perhaps difficulties in the way of the proofs 

 of their existence, because the egg-cell lies free for a long time 

 and even moves actively in the tissue of the mesogloea. We 

 might expect that the formation of polar bodies takes place 

 here, as in all other instances, when the ^g% becomes mature, 

 that is, at a time when the eggs are already closely enveloped 

 in the sponge tissue. At all events the eggs of sponges, as far 

 as they are known, attain a specific nature, in the possession of 

 a peculiar cell-body, frequently containing 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 nucleoplasm, 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 Hoek and in Cetochihis septentrionale by Grob- 

 ben. The latter instance appears to be quite trustworthy, but 

 there is some doubt as to the former and also as regards Moimi 

 (a Daphnid), in which Grobben found a body, which he con- 

 sidered to be a polar body, on the upper pole of an t.gg 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 Petrouiyzon 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 ovogenetic 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 



' The existence of polar bodies in sponges has been recently proved 

 by Fiedler: Zool., Anzeiger., Nov. 28, 1887.— A. W., 1888. 



'•* They have now been observed in many species, so that their general 

 occurrence in insects is tolerably certain. Compare bibliographN' given in 

 Weismannandlschikawaj'W^eitereUntersuchungenzumZahlengesetz der 

 RichtungskOrper,' 'Zoolog. Jahrbucher,' vol.iii. 1888, p. 593. — A.W., 1888. 



