42-2. THE HISTORY OF BIOLOGY 



does not detract very much from the value of the service he rendered in 

 throwing light on an important field in the biology of the lower animals. 



Leuckart's discovery of micropj/le 

 Leuckart made another valuable contribution to the development of bi- 

 ology in his discovery that the thick-shelled egg of insects is invariably 

 provided with a canal through which fertilization takes place. This canal, 

 which Leuckart, associating it with a corresponding formation in the vege- 

 table kingdom, called "micropyle," was studied by him with great thor- 

 oughness in a large number of eggs of various insects. His investigation led 

 him to the discovery that the spermatozoa actually do penetrate into the 

 yolk of the egg through the canal — a discovery that essentially deepened 

 our knowledge of fertilization. That the spermatozoa thus play a vital part 

 in fertilization was a point which many investigators have had difficulty in 

 realizing; after all, it was not so long ago that Spallanzani's theory of the 

 decisive importance of the spermatic fluid had eminent supporters. J. Miiller, 

 it is true, had already discovered a similar canal in the eggs of the sea-urchin, 

 but it was Leuckart who proved its widespread existence and thereby also 

 its significance. Henceforth there was no doubt that the spermatozoon pro- 

 duced fertilization by penetrating the egg; it was then reserved for the fu- 

 ture to ascertain the cytological course of development. 



Of Leuckart's other works should be mentioned his investigations of 

 the Spongida, which he referred to the Coelenterata as a result of detailed 

 study of their structure. Further, his experiments on the intestinal worms, 

 conducted in competition with Siebold; it was Leuckart who found out the 

 evolutional process in the two well-known human parasites Tania solium 

 and saginata, as also in the liver-fluke, which is so fatal to domestic animals. 

 It was also thanks to him that the Trichinas first became thoroughly known. 

 His important text-book on the human parasites is the work on which all 

 subsequent research in this field has been based. 



A peculiar form of parasitism is presented by the crustaceans that make 

 fishes their hosts; in these the parasitic degeneration assumes forms such as 

 scarcely exist anywhere in the rest of the animal kingdom. The first real 

 knowledge of these animal forms was established by Alexander von Nord- 

 MANN. He was born in 1805 of a Germanized Finnish family in the county 

 of Wiborg, Finland, studied at Abo and from there went to Berlin, where 

 he became a pupil of Rudolphi. There he wrote his work Mikrographische 

 Beitrage, in which he deals with certain parasitic Trematoda, but chiefly 

 with the parasitic Crustacea, which were hereby brought to the knowledge 

 of science. The work attracted universal notice; J. Miiller in a letter speaks 

 of its " herrliche Beobachtungen," and on account of it the author was called 

 to a professorial chair in Odessa. There he applied himself to exploring the 

 animal world of South Russia, both recent and extinct, which he described 



