34 COMPARATIVE ANATOMY CHAP. 



Abnormally, two or more spermatozoa may enter an egg. In sue** 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 (cf. 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 I 

 and female. 



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

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

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

 possess special organs of copulation. 



Various theories about the nature of fertilisation have been recently brought I 

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

 Geddes and Thomson, and others. 



Literature. 

 Comprehensive Works. 



Besides Balfour's Comparative Embryology, consult especially : 

 O. Hertwig. Lehrbuch der Entwickelungsgeschichte des Menschen und derm 



Wirbelthiere. 3d edition. Jena, 1890. 

 W. Waldeyer. Eierstock und Ei. Leipzig, 1870. 



The same. Ban und Entwickelung der Samenfdden. Anat. Anzeiger. Jena, 1887 J 

 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 thej 

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

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

 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 ori 

 Histology. How different cells and complicated tissues arise out oq 

 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 j 

 Protozoa as widely spread asexual reproductive processes. Whereas,! 

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

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



