8 



the final subdivision of germ-cells, retained unchanged at 

 the filamentary extremities of the branched uterus forming 

 the ovaria of the larval Aphides. 



In most of the lower classes of animals the course of de- 

 velopment is temporarily arrested at certain stages, though 

 growth may go on ; the embryo moving and feeding, and 

 perhaps propagating, as if it were a completed individual, 

 usually under a form very different from that which itself 

 or its progeny are destined ultimately to assume ; whence 

 these arrested forms have been termed e Larva?/ the true 

 lineaments of the fully developed form being hidden, as it 

 were, beneath a mask. 



The earlier the individual in any of these larval stages 

 may have been arrested from the commencement of its 

 development from the germ-mass, the greater is the pro- 

 portion of the derivative impregnated germ-cells and nuclei 

 that continue unchanged in its constitution : and the result 

 of the retention of these, in the hydriform larvae of Aca- 

 lephes, e. g., is the exercise, as in the mature freshwater 

 Hydra, by one or more of such retained progeny of the 

 primary impregnated germ-cell, of the powers derived from 

 the legacy of the portion of the spermatic virtue which they 

 received from their parent nucleated cell. 



In the Polygastria, e.g. when favourable influences of 

 warmth, light and abundant nutriment concur, a central 

 body, which represents the nucleus of the impregnated 

 germ-cell, sets on foot the special act of assimilation and 

 spontaneous fission ; and its divisions seem to repel each 

 other to positions equidistant from each other, and from 

 the pole or end of the body to which they are nearest. 



The influence of these distinct centres of assimilation is 

 to divert the flow of the plasmatic fluid from a common 

 course through the body of the Polygastrian to two special 

 courses about those centres. So much of the primary de- 

 velopmental processe is renewed, as leads to the insulation 

 of the sphere of the influence of each assimilative centre 



