MODERN BIOLOGY 535 



be made to Theodor Boveri (1861-1915), a disciple of the brothers Hertwig 

 and professor at Wiirzburg, as well as the two Heidenhains — Rudolf 

 (1834-97), a pupil of Ludwig and professor of physiology at Breslau, but 

 active also as a cytologist, and his son Martin, born in 1862. and professor 

 at Tubingen, who devoted himself exclusively to cell research. All the above 

 have advanced their science by making valuable discoveries and important 

 technical improvements. 



Strasburger on the formation and division of cells 

 In 1875 "^^^ published the first edition of Strasburger's pioneer work Zell- 

 bildung und Zellteilung, a third completely revised edition came out in 1880. 

 The main problem that occupied cytological research during this period was 

 that of the origin of the cellular nucleus. As we have seen, Nageli had al- 

 ready observed the division of the nucleus, but neither his own nor other 

 similarly extensive observations were able to possess general application. 

 Even in the first edition of his said work Strasburger makes the nucleus of 

 the egg-cell in the plants he investigated dissolve upon fertilization and its 

 mass disperse into the plasm of the cell; in the latter are then formed a 

 number of concretions, which give rise to fresh nuclei. In the third edition, 

 on the other hand, it is asserted that examples of independent cell-formation 

 can no longer be cited from the vegetable kingdom; fresh nuclei invariably 

 arise through the division of older ones. This established one more of the 

 principles of modern cytology. Even before this students had begun to ob- 

 serve the curious phenomena attending nuclear division in the majority of 

 cells, but apart from these scattered observations, it was Strasburger who, 

 as far as the vegetable kingdom is concerned, elucidated this process, which, 

 though complicated, is now widely known and is set forth in all text-books. 

 This process — indirect nuclear division, also called "mitosis" or "kary- 

 okinesis" — is as follows: the nucleus, having lost its membrane, concen- 

 trates its colourable contents around its middle plane, after which the latter 

 divides itself and the two halves go each its own way and thereupon again 

 concentrate into two daughter-nuclei. The main principles of this process 

 were already elucidated in the above-mentioned first edition of Strasburger's 

 book, and in the third a number of further details are given. In the field of 

 zoology Biitschli, O. Hertwig, and Flemming during the same decade made 

 their decisive contributions to our knowledge of nuclear division, and, be- 

 sides, certain isolated details were discovered by Fol, van Beneden, and 

 others. As a result of this research work the elements composing the nucleus 

 were also investigated; filament substance and nucleolus, nuclear juice, and 

 nuclear membrane were the constituents that were distinguished to begin 

 with. Of these the first-mentioned was, owing to the part it plays in the 

 nuclear divisions, the object of greatest attention, and especially on this 

 subject Flemming's studies of the cells of amphibious larva; were conclusive. 



