i EGG CELLS, SPERM CELLS 23 



R. Hertwig. Der Organismus der Radiolarien. Jena, 1879. 



Ch. G. Ehrenberg. Die Infusionsthiere als vollJcommene Organismen. Leipzig, 1838. 



Claparede et Lachmann. Etudes sur Us Infusoires et Us Ehizopodes. .Geneve, 1858- 



1859. 



Fr. Stein. Der Organismus der Infusionsthiere. I. -III. Leipzig, 1859, 1867, 1879. 

 0. Biitschli. Ueber die Conjugation der Infusorien, in: Studien uber die ersten 



Entwickelungswrgange, etc. Frankfurt, 1876. 

 Saville Kent. A Manual of the Infusoria. London, 1880-1882. 

 Dujardin. Histoire naturelle des Infusoires, in : Suites a Buff on. Paris, 1841. 

 Carpenter. Introduction to the Study of the Foraminifera. London Ray Soc. , 1862. 

 J. Miiller. Ueber die Thalassicolen, Polycystinen und Acanthometren. Abhandl. der 



Berliner Akadcmie, 1858. 

 K. Brandt. Die koloniebildenden Radiolarien (Sphaerozoeen) des Golfes von Neapel. 



Berlin, 1885. 

 0. Biitschli. "Protozoa," newly edited, in Bronn's Klassen und Ordnungen des 



Thierreichs. I. Band. Not yet completed. 



Balbiani. Various treatises in the Journal de TAnatomie et de la Physiologic. I. -IV. 

 Various important treatises of Haeckel, Hertwig, Schneider, Engelmann, Maupas, 



F. E. Schulze, Greef, Bergh, Czienkovsky, Jikeli, etc. 



Egg Cells, Sperm Cells, Fertilisation, Sexual Reproduction 

 of the Metazoa. 



Whereas Protozoa are either simple cells or colonies of similar 

 cells, all other animals, or Metazoa, appear as complicated communities, 

 the individual cells of which are no longer similar. Division of 

 labour arises among the cells; every cell (or group of cells) in 

 the community having to fulfil only one special function is con- 

 stituted in correspondence with this function (polymorphism of the 

 cells of a Metazoan colony). However wonderfully complicated such 

 a cell community may be, it always develops (except in cases of 

 asexual reproduction) by means of the continuous division of one 

 single cell, the fertilised egg. This is the product of the fusing of 

 a female reproductive cell with a male reproductive cell, i.e. it is 

 the result of fertilisation. Reproduction by means of such sexually 

 differentiated reproductive cells is called sexual reproduction. In 

 all forms of Metazoa (with a few not quite certainly established 

 exceptions) sexual reproduction occurs at least at times, and con- 

 stitutes an essential characteristic by which Metazoa are distinguished 

 from Protozoa. It is true that we found in the latter the beginnings 

 of sexual reproduction. As among the Protozoa a series of phenomena 

 lead up to the sexual reproduction of the Volvox colony, so the 

 latter is directly connected with the simplest form of the sexual 

 reproduction of the Metazoa. 



In sexual reproduction we have to bear in mind two distinct points : 



1. The fusing of the cells, or more accurately of two cell 



nuclei, which here takes place a phenomenon which is analogous 



to the processes of copulation and conjugation in the Protozoa. The 



