34 COMPARATIVE ANATOMY chap. 



Abnormally, two or more spermatozoa may enter an egg. In such a case several 

 male pronuclei may fuse with the female. Compare on this point the works of Fol and 

 Hertwig. It is not improbable that twin- and triple-formations may be produced 

 by such over - fertilisation. The development after over - fertilisation in all cases 

 deviates characteristically from the normal course. 



The most essential morphological characteristic of fertilisation is 

 the fusing of two sexually differentiated cell nuclei, the male and the 

 female pronuclei. In the conjugation of the Protozoa (Paramcecium), 

 also we have to do (c/. p. 18) with a fusing of two nuclei (the stationary 

 and the migratory nucleus). Fertilisation in this latter case, however, 

 is mutual, and we cannot distinguish the conjugating cells as male 

 and female. 



Fertilisation is either internal, i.e. takes place ^vithin the mother body, or 

 external, i.e. spermatozoa and eggs are expelled from the parental bodies and meet 

 each other outside in water. In the first case at least the reproducing animals 

 possess special organs of copulation. 



Varioiis theories about the nature of fertilisation have been recently brought 

 forward, especially by Biitschli, Balfour, Sabatier, van Beneden, Hertwig, Weismann, 

 Geddes and Thomson, and others. 



Literature. 

 Comprehensive Works. 



Besides Balfour's Comparative Emhryology consult especially : 

 0. Hertwig. Lehrhuch der EntwickehmgsgescMchte cles Menschen und der 



Wirlelthiere. 3d edition. Jena, 1890. 

 W. Waldeyer. Eier stock und Ei. Leipzig, 1870. 



The same. Bau und Entwickelung der Samenfaden. Aaat. Anzeiger. Jena, 1887, 

 where complete bibliography is given. 



Tissue Cells and Cell Tissue. 



We have till now considered (1) unicellular organisms, and (2) the 

 egg- and sperm-cells, which fuse to form the starting point in the 

 individual development of all the higher, i.e. multicellular, animals. 

 We will now briefly deal with the manner in which the Metazoan 

 body is composed of cells, and consider the various cells of which 

 it consists. The observation of the cells of the animal body and of 

 their complexes, the tissues, is the object of the science of tissues or 

 Histology. How different cells and complicated tissues arise out of 

 simple indifferent cells, is the subject of Histogeny. 



As our first principle we can state that all cells and tissues of the 

 adult animal body arise by means of repeated division from the 

 fertilised egg -cell. These phenomena of division are the same as 

 those with which we became acquainted among the unicellular 

 Protozoa as -widely spread asexual reproductive processes. Whereas 

 however, in most Protozoa the products of division separate, and, like 

 the mother cell, lead an independent life, in the Metazoa the descend- 



