rOLYGASTRIA. 31 



of the confluence of the parts around it, and to call it " a centre of 

 attractive and assimilative force." * Since the pellucid centre of the 

 germinal body has not divided from the necessity of endowing the 

 moiety to be separated by the subsequent fission with a particular 

 organ required for its individual completeness, I infer that the same 

 preliminary act in the monad was not solely for the purpose of pro- 

 viding its separated moieties with their respective testes, but that it 

 had a higher significance. 



As the pellucid centre in the ovum is the result of impregnation or 

 the reception of the matter of the spermatozoon, so it may be con- 

 cluded that the nucleus of the monad is of a nature similar to, if 

 not identical with, that of the spermatozoon. It was doubtless a 

 gross view of its nature and analogies to regard it as the homologue 

 of the whole preparatory organ of the spermatic fluid, such as is re- 

 quired in the higher animals ; because as the germ-cells exist in the 

 body of the Polygastria without the organ called ovarium, so we 

 ought to expect that the essential matter of the sperm would likewise 

 exist without a special testicular envelope. 



The objection, however, to Ehrenberg's determination of the 

 nucleus as the " testis," that it has never been observed to produce 

 spermatozoa, is akin to that which has been opposed to his determin- 

 ation of the ova, viz. that the young have never been seen to quit 

 them and leave the shell behind. Neither of these objections will 

 apply to the view of the nucleus as the essential matter of the 

 sperm, and of the germ-cells as the essential elements of ova and 

 embryo. 



A spermatozoon is doubtless a very general form of the essential 

 matter of the sperm : but in tracing the modifications of the sper- 

 matozoa from mammalia down the scale of animal life, we find them 

 gradually reduced to the head or nuclear part, and discern in the 

 vibratile caudal appendage an accessory relating to the passage of tlie 

 fertilising principle to the germ-cell, rather than to its essential opera- 

 tions when arrived there. 



The best microscopical examinations of the spermatozoa show that 

 they consist of a homogeneous or minutely granular substance, which 

 exhibits a yellow amber-like glitter. The nucleus of the Polygas- 

 trian offers the closest reseml)lance to this character of tissue. And 

 it is not uninteresting to notice the close analogy of the modification 

 of form which the nucleus of some of the larger Polygastria, Stetitor 



XX I X. p. 203. XXX. p. cr,. 



