232 COMPARATIVE ANATOMY 



of fission of a zygote separate from one another, lead free 

 individual existences, and form a stock or family which has a 

 limited term of existence, for its members degenerate and die 

 if they do not conjugate at the proper time. In a Metazoon 

 the product of fertilisation is the fertilised ovum or oosperm, 

 comparable to the zygote of a Protozoon. The oosperm 

 reproduces itself, just as Protozoa reproduce themselves, by 

 binary division, but its progeny, instead of separating, remain 

 adherent, forming an integral whole or individual. Countless 

 numbers of cells are formed by repeated binary division, and 

 the whole, of which they form a part, grows into what we call 

 the adult organism. This we may compare with the sum of the 

 numerous individuals forming a stock or family of Protozoa. 

 Even as the Protozoon family has a limit of existence, degene- 

 rating and eventually dying if that limit is overpassed ; so the 

 Metazoon body, composed of the innumerable progeny of the 

 oosperm, has its well-marked phases of growth, maturity, decay, 

 and death. I In both cases the extinction of the race is provided 

 against by the act of conjugation or fertilisation, by means of 

 which the forces of life to use a vague expression are as it 

 were rejuvenated and started on a new cycle of activity by the 

 commingling of the nuclear material of two simple cells. 

 Further than this we see a close parallel in the manner in 

 which the nuclear material is halved, before fertilisation, by 

 the formation of the polar bodies, or by the division of the 

 spermatocytes, in the Metazoan ovum and spermatozoon and the 

 similar maturation divisions in Actinosphserium, Copromonas, 

 and Paramecium. The alternation of generations, so obvious 

 in many Protozoa, would seem to be masked in the Metazoa 

 by the coherence and integration of the cell progeny of the 

 fertilised ovum. 



Thus the ovum and spermatozoon might be regarded as the 

 sexual generation of cells, the tissue-cells derived from them 

 as the numerous asexual generations, and the cycle is completed 

 on the reappearance of the ovum and spermatozoon. Such 

 comparisons open up a wide field of inquiry, the interest in 

 which is further increased when one studies the parallel series 

 of events in plants ; and the gradations exhibited by Pandorina, 

 Eudorina, and Volvox seem to supply a real basis to specula- 

 tion. But at present the whole subject is too obscure to be 

 discussed with profit. It must not be forgotten that no pro- 



