THE OKIGIN OF SEX. 35 



In the colonial Protozoa, such as Volvox, which take the 

 form of hollow balls of cells, certain of the cells become large 

 and stationary, forming the female cells or ova ; these are 

 fertilised by small active male cells, derived from the same or 

 from other colonies ; and then, by division of the fertilised ova, 

 new balls or colonies are formed. 



This process is essentially the same as the sexual reproduc- 

 tion of Metazoa, and there can be little doubt that the process 

 has been inherited by the Metazoa from their Protozoan ancestors. 



The reason for the occurrence of sexual reproduction in all 

 Metazoa is probably to be found, as suggested above, in the 

 consideration that it is through sexual reproduction alone that 

 the full advantage of cross-fertilisation can be obtained. This 

 view, that sexual reproduction is to be regarded as highly 

 advantageous rather than as absolutely essential to the species, is 

 of great importance, as it leaves room for, and renders intelligible, 

 the occurrence of other and asexual modes of reproduction such 

 as are seen in so many groups of Invertebrates. It also affords 

 a clue to the extraordinary condition of things described in 

 certain of the pelagic Tunicates. Salensky has shown that in 

 Salpa, and to a less marked degree in Pyrosoma, certain of the 

 follicle-cells surrounding the ovum pass into its interior, and 

 take an active part in the formation of the embryo ; so that, 

 although the egg is fertilised in the ordinary manner, the blasto- 

 meres resulting from its segmentation only give rise to certain of 

 the component cells of the embryo, and not, as is usually the case, 

 to all of them. This mode of development may be regarded as 

 a combination of the ordinary sexual process with an asexual 

 process similar to that by which the geminules of sponges or the 

 statoblasts of Polyzoa are formed. 



List of the more important Books and Memoirs bearing on 

 the Subjects of Chapter I. 



Balfour, F. M. : ' Treatise on Comparative Embryology.' Vol. i. chaps, i. ii. iii. ; 



vol. ii. chap. xiii. 1880-81. 

 Beneden, E. v. : ' Recherches sur la maturation de 1'oeuf et la fecondation.' 



Archives de Biologie, iv. 1884. 

 Beneden, E. v., et Neyt, A. : ' Nouvelles recherches sur la fecondation et 



la division mitotique chez 1'Ascaride megalocphale.' Bulletin de 



1' Academic Eoyale des Sciences de Belgique, 3 e ser., tome xiv. 1887. 



D 2 



