254 



REDUCTION OF THE CHROMOSOMES 



In the first of his classical cell-studies Boveri ('87, i) reached the 

 same result through a careful* study of Ascaris megalocepJiala, showing 

 that each tetrad appears in the germinal vesicle in the form of four 

 parallel rods, each consisting of a row of chromatin-granules (Fig. 1 17, 

 A-C). He believed these rods to arise by the double longitudinal 

 splitting of a single primary chromatin-rod, each cleavage being a 



Fig. 126. Tetrad-formation in an insect, Anasa. [PAULMIER.] . 



A, Resting spermatogonium with single plasmosome and two chromatin-nucleoli. B. Equa- 

 torial plate of dividing spermatogonium ; twenty large and two small chromosomes. C. Final 

 spermatogonium-division. D-I. Prophases of first maturation-division. D. E. Synapsis, with 

 single chromatin-nucleolus. F. Segmented split spireme. G. H. Formation of the tetrad-rings. 

 H. I. Concentration of the rings to form tetrads. 



preparation for one of the polar bodies. In his opinion, therefore, 

 the formation of the polar bodies differs from ordinary mitosis only 

 in the fact that the chromosomes split very early, and not once, but 

 twice, in preparation for two rapidly succeeding divisions without an 

 intervening resting period. He supported this view by further obser- 

 vations in 1890 on the polar bodies of Sagitta and several gastero- 

 pods, in which he again determined, as he believed, that the tetrads 



