i EGG CELLS, SPEEM CELL* 23 



R. Hertwig. L)er Organisms dcr Radiolaricn. Jena, 1879. 



Ch. G. Ehrenberg. Die Infusionsthicrc als wllkommene Organismen. Leipzig, 1838. 



Claparede ct Lachmann. titudcs sur Ics Infusoircs d Ics Rhizopodes. Geneve, 1858- 



1859. 



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

 0. Biitschli. Ueber die Conjugation der Infusorien, in : Studicn iibcr die erstcn 



Entwwkelungswrgiinge, etc. Frankfurt, 1876. 

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

 Dujardin. Histoire naturellc des Infusoires, in : Suites d Buffon. Paris, 1841. 

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

 J. Miiller. Ueber die Thalassicolcn, Polycystinen und Acanthometrcn. Abhandl. </< / 



Berliner Akademie, 1858. 

 K. Brandt. Die kolonielildenden Radiolaricn (Sphacrozoecn) des Golfes von Neapcl. 



Berlin, 1885. 

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



Thierreichs. I. Band. Not yet completed. 



Ealbiani. Various treatises in the Journal de I' Anatomic et de la Physiologic. I. -IV. 

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



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



Egg 1 Cells, Sperm Cells, Fertilisation, Sexual Reproduction 



of the Metazoa. 



AVhereas 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 esta.blish.ed 

 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 



