44 INVERTEBRATE MORPHOLOGY. 



As a mode of reproduction in the Metazoa division plays 

 but a secondary part, the sexual process being the character- 

 istic method. Attention has already been called to the par- 

 tial specialization in Volvox of reproductive cells which serve 

 to perpetuate the species, the remaining cells of the colony 

 perishing. This condition is a premonition of the more per- 

 fect specialization found in the Metazoa of reproductive or germ 

 cells and non-reproductive or somatic cells, the latter serving 

 for the nutrition and protection of the germ-cells, to which 

 the perpetuation of the species is entrusted. Comparatively 

 early in the development of an individual certain cells differ- 

 entiate from the others, not undergoing like them a physi- 

 ological and structural specialization, but retaining a general- 

 ized character. These are the germ-cells usually grouped 

 together to form the reproductive organs. 



In describing the methods of reproduction occurring in the 

 Flagellata, the manner of the development of sexual repro- 

 duction was indicated. It appears to have been originally a 

 more or less accidental fusion of two similar cells or spores, 

 and from being accidental this fusion gradually became the 

 rule on account of the greater vitality which the conjugate in- 

 dividual possessed over cells which did not conjugate. The 

 next step was the differentiation of rnicrospores and macro- 

 spores, which reaches a high development in Volvox, where 

 it is associated also with a differentiation into somatic and 

 germ cells. In the Metazoa both these differentiations are 

 carried to a higher degree, the niacrospores being known 

 as ova and the microspores as spermatozoa, while the aggre- 

 gates of these cells are termed respectively ovaries and testes. 



In a young embryo a mass of germ-cells which is to give 

 rise to spermatozoa cannot be distinguished from one which 

 is destined to be converted into ova. Fundamentally both 

 are the same, and occasionally a portion of a mass of germ- 

 cells may be differentiated into ova, while the rest of it devel- 

 ops into spermatozoa. This has not unfrequeutly been seen 

 in fishes in which there is normally a separation of the sexual 

 elements in distinct individuals, and throws considerable 

 light upon the occurrence of forms which normally possess 

 both elements. This condition of hermaphroditism, which oc- 



