408 LECTURE XVIII. 



either by coalescing with others or by liquefaction, they do not lose 

 their vitality ; as individuals, indeed, they may be said to die, but by 

 their death they minister to the life of a being higher than themselves ; 

 they combine to construct its tissues, or dissolve and impart properties 

 to its fluids ; these metamorphoses being mysteriously governed by 

 a plastic nature or mode of force operating unconsciously upon 

 the matter, but according to a law of order and harmony, and 

 directed to a fore-ordained and definite end, resulting in a distinct 

 and specific form of animal, adapted by its organisation for a par- 

 ticular sphere of existence, and forming a more or less valuable, but 

 not, as once was thought, an essential link in the great chain of 

 organic life. 



It is important, however, to bear in mind, that not all the progeny 

 of the primary impregnated germ-cell are required for the formation 

 of the body in all animals : certain of the secondary germ-cells, or 

 their nuclei, may remain unchanged, and become included in that 

 body which has been composed of their metamorphosed and diversely 

 combined or confluent brethren. So included, any such cell, or its 

 nucleus, may commence and repeat the same processes of growth by 

 imbibition, and of propagation by spontaneous fission, as those to 

 which itself owed its origin ; followed by metamorphoses and com- 

 binations of the cells so produced, which concur to the development 

 of another individual ; and this may be, or may not be, like that in 

 which the secondary germ-cell was included. 



In the previous Lectures we have seen that, in proportion as the 

 subjects of anatomical investigation descend in the scale of animal 

 life, the number of the derivative nucleated cells which retain their 

 individuality and spermatic power is greater, and the number of 

 those that are metamorphosed into tissues and organs less (p. 35). 



A large proportion of such impregnated cells is retained un- 

 changed in the compound hydriform Polypes and in the parenchy- 

 matous Entozoa : a smaller proportion in the Acalephce and cavitary 

 Entozoa. We find derivative germ-cells and masses of nuclei, like 

 those resulting from the final subdivision of germ-cells, retained 

 unchanged at the filamentary extremities of the flabelliform uterus, 

 and forming the ovaria of the larval Aphides. By the observation 

 of this phenomenon in the newly hatched larval Aphis from the 

 ovum deposited by the oviparous species, and by reflection on the 

 relation of the observed germ-masses to the successive spontaneous 

 fissions of the primary impregnated germ-cell, and to the effect of 

 such spontaneous fissions in the subdivision and diffusion of the 

 spermatic force, I arrived, some years ago, at what I felt to be a clear 



