66 



force have been set up in place of one, and the plastic power 

 has arranged the organic matter around each of these cen- 

 tres, as it was around the single nucleus of which they are 



of the germ-yelk has not been seen in the ova of other animals, it is be- 

 cause hitherto only the coarser phaenomena of such division of the ger- 

 minal body in Medusae, Mollusca, Fishes, Frogs, &c. have been noticed. 

 In Dr. Barry's observations however on the development of the germ- 

 mass in the pellucid ova of the rabbit, phenomena closely analogous 

 to those described by Siebold and Bagge in the ovum of the entozoon 

 were detected. 



In reflecting on the phsenomena in the monad and the ovum that 

 a central something is first established, and the consequence of that 

 I have been led to draw the same conclusion with respect to both, 

 and to regard the establishment of the special centre as the cause of the 

 confluence of the parts around it ; and I 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 sepa- 

 rated 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 providing 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 of 

 the reception of the matter of the spermatozoon, so it may be concluded 

 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 pre- 

 paratory organ of the spermatic fluid, such as is required 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 essen- 

 tial matter of the sperm would likewise exist without a special testicular 

 envelope. 



The objection 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 determination 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 to the germ -cells as the essential 

 elements of ova. 



A filamentary spermatozoon is doubtless a very general form of the 

 essential matter of the sperm : but in tracing the modifications of the 

 spermatozoa 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 the 



