HISTORICAL SKETCH 15 



be a complete cell with its nucleus and cytoplasm, as von Kolliker had 

 maintained. That Schwann (1839) had been right in considering 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 as the result of 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 (b. 1849) announced the important discovery that the two 

 nuclei seen fusing in the fertilized egg are furnished by the egg and the 

 spermatozoon by the two parents. The role of the nucleus in fertiliza- 

 tion was thus demonstrated 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 fertilization in both kingdoms consists in the union of 

 two cells (gametes), one from each parent (in dioscious forms), and that 

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

 new individual therefore deriving half of its nuclear substance from each 

 parent. 



Although the cleavage of the fertilized animal egg to form 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 known to be 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, 

 and by a number of later writers, including A. von Kolliker, who traced 

 in detail the long series of changes by which the multiplying embryonic 

 cells become differentiated into the various tissues and organs. Embry- 

 ogeny was thus shown to be a process of cell-division and differentiation, 

 the fertilized egg cell initiating a series of divisions giving rise to all the 

 cells of the body, and to the germ cells. The life cycle was now recognized 

 as a cell cycle ; and since the egg is the direct descendant of the egg of the 

 previous generation it became evident, as Virchow pointed out in 1858, 

 that there has been an uninterrupted series of cell-divisions from the 

 beginnings of life on the earth in the remote past down to the organisms 

 in existence today. The statement of this conception is known as the 

 Law of Genetic Continuity. In the words of Locy (1915) : 



"The conception that there is unbroken continuity of germinal substance 

 between all living organisms, and that the egg and the sperm are endowed with an 

 inherited organization of great complexity, has become the basis for all current 

 theories of heredity and development. So much is involved in this conception 

 that ... it has been designated (Whitman) 'the central fact of modern 



