438 INTRODUCTION TO CYTOLOGY 



into the egg, but it was thought at the time and for many years afterward 

 that they were parasitic animalcules in the spermatic liquid; hence the 

 name "spermatozoa." 



Although L. Spallanzani (1786) is usually said to have shown by a 

 filtration experiment that the spermatozoon is the fertilizing element, 

 it is pointed out by Lillie (1916) that Spallanzani did not draw the 

 correct conclusion; he even denied that the spermatozoon is the active 

 element, holding rather that the fertilizing power lies in the spermatic 

 liquid. It was Prevost and Dumas who corrected this mistake and 

 demonstrated the true role of the spermatozoon (1824). The spermato- 

 zoon was later shown by Schweigger-Seidel (1865) and La Valette St. 

 George (1865) to be a complete cell with its nucleus and cytoplasm, as 

 von Kolliker had maintained. That Schwann (1839) had been right in 

 regarding the egg as a cell was shown by Gegenbaur in 1861. The polar 

 bodies formed at the time the egg matures are said to have been first 

 seen by Carus (1824). Biitschli (1875) showed them to be formed in 

 connection with the division of the egg nucleus, and Giard (1877) and 

 Mark (1881) interpreted them as abortive eggs. 



The penetration of the spermatozoon into the egg was not actually 

 seen until Newport (1854) observed it in the case of the frog. In 1875 

 O. Hertwig (1849-1922) announced the important discovery that the two 

 nuclei which fuse in the fertilized egg are furnished by the egg and the 

 spermatozoon. The role of the nucleus in syngamy was thus demon- 

 strated in animals only shortly before it was in plants, and it is interesting 

 to note that the first complete description of the union of the germ cells 

 in animals was given by H. Fol in the same year (1879) that Schmitz 

 described clearly the process in plants. It was now evident that syngamy 

 in both kingdoms consists in the union of two gametes which are ordi- 

 narily single cells, one from each parent (in dioecious forms), and that the 

 central feature of the process is the union of two gamete nuclei, the new 

 individual therefore deriving a portion of its nuclear substance from 

 each parent. 



Although the cleavage of the fertilized animal egg in the development 

 of the embryo had been seen many years previously, it was first definitely 

 described by Prevost and Dumas in 1824 for the frog. At that time 

 neither the egg nor the products of its division were clearly recognized as 

 cells. The true meaning of cleavage was elucidated by M. Barry, who 

 held that the blastomeres are cells and that their division is preceded by 

 the division of their nuclei; also by a number of later writers, including 

 A. von Kolliker, Whitman and Rabl, who traced in detail the long series 

 of changes undergone by the multiplying embryonic cells as the various 

 tissues and organs are differentiated. Embryogeny was thus shown to 

 involve the division and differentiation of cells, the fertilized egg initiating 

 a series of divisions giving rise to all the cells of the body and to the germ- 



