14 INTRODUCTION TO CYTOLOGY 



lowed the process with his characteristic thoroughness there could remain 

 no doubt concerning the error of Schleiden and Schacht. Hofmeister 

 clearly demonstrated that the embryo arises, as Amici contended, not 

 from the end of the pollen tube, but from an egg contained in the ovule, 

 the egg being stimulated to development by the pollen tube. He was 

 wrong, however, in supposing that the tube did not open, but that a 

 fertilizing substance diffused through its wall. 



It was in the algse that the union of the sperm cell with the egg cell 

 (the act of fertilization) was first seen in the case of plants. In 1853 

 Thuret saw spermatozoids attach themselves to the egg of Fucus, and in 

 1854 he showed that they are necessary to its development. The actual 

 entrance of the spermatozoid into the egg was first observed in 1856 by 

 Nathanael Pringsheim (1824-1894) in (Edogonium. The fusion of the 

 parental nuclei was seen by Strasburger (1877) in Spirogyra, but he 

 thought they thereupon dissolved. This error was corrected shortly 

 afterward by Schmitz (1879), who was thus the first to show clearly that 

 the central feature of the sexual process in plants is the union of two 

 parental nuclei to form the primary nucleus of the new individual. 



That the same process occurs in fertilization in the higher plants 

 was demonstrated by Strasburger, who in 1884 described the union of the 

 egg nucleus with a nucleus brought in by the pollen tube. In 1898 and 

 1899 S. Nawaschin and L. Guignard completed the story by describing 

 the phenomenon of double fertilization, whereby the second male nucleus 

 contributed by the pollen tube unites with the two polar nuclei to form 

 the primary endosperm nucleus. The subsequent work of Strasburger 

 and others on the gymnosperms and angiosperms greatly cleared up the 

 whole matter of fertilization and embryogeny in these plants. This 

 work belongs to the modern period of cytology. 



In Animals. It is probable that the spermatozoon was first seen in 

 1677 by Ludwig Hamm, a pupil of Leeuwenhoek. The credit for the 

 discovery, however, is usually given to Leeuwenhoek, since it was he who 

 brought the matter to the attention of the Royal Society and pursued 

 such studies further. He asserted that the spermatozoa must penetrate 

 into the egg, but it was thought at that time and for many years after- 

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

 the name "spermatozoa." 



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

 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 spermatozoon was later 

 shown by Schweigger-Seidel (1865) and La Valette St. George (1865) to 



