438 On Anvpullaceous Sacs and Spongozoa. 



and if right, then we see the resemblance of the ampullaceous 

 sac to the Ascidian, more especially of the compound Tunicata, 

 in so far that each has an oral and an anal aperture. 



" Very good," it may be stated ; " but what have you in the 

 Ascidian to compare to your individualized spongozoa, each of 

 which appears to be a distinct animal? " 



In reply to this I would observe that if among the com- 

 pound Tunicata (which, being of the same habitat as Halisarca^ 

 grow plentifully together) we select a species that, by its 

 translucence, permits of the young or embryonal Ascidians of 

 the group being viewed under a microscope throughout their 

 development, we may see that, at one stage, the young Ascidian 

 is almost identical in appearance with the ampullaceous sac — 

 that is, that it is composed of a pavement of cells aggregated 

 into a sac-like form. Finally the internal organs of the 

 Ascidian are developed, and the sac acquires an oral and an anal 

 aperture. But in the sponge — that is, in Halisarca (as we are 

 now more particularly engaged with this sponge) — the cells of the 

 pavement-layer pass into individualized animals, each of which 

 takes in its proper nutriment, and probably possesses indi- 

 vidually or conjointly its proper generative function ; for to 

 assume that they do not possess the latter would be to assume 

 that the isolated forms described and figm*ed by the late Prof. 

 James-Clark (' Annals,' 1868, vol. i. p. 133, pi. v.) do not 

 possess this power. 



Thus the development of the ampullaceous sac is arrested, 

 and the cells adapt themselves to that condition which ends in 

 the evolution of a sponge ; while the sac and its cells in the 

 development of the Ascidian go on to the evolution of a 

 compound tunicated group. 



To make this more intelligible it might be stated that if, 

 by the theory of evolution, the monad passes up to man, the 

 monadine cell could not, at the time of reproduction, have 

 been identical with the human ovum, because it must have 

 possessed specialized powers of self-nutrition and immediate 

 generation. 



At the same time the differentiation that takes place in the 

 evolution of the variously formed cells composing respectively 

 the organs of the human body, from the simple cell-form of 

 the human ovum, points out differences almost as great if 

 not as numerous as those in the animals between man and the 

 monad. 



Hence we see that an organic cell may be at one time one 

 thing and at another another, arising simply from adaptation 

 to the functions required. Thus the cell or spongozoon of 

 the ampullaceous sac becomes an individualized animal, 



