ORIGIN OF THE TETRADS 1 8/ 



four equivalent chromosomes, but of two different pairs ; and the 

 second or transverse division by which a is separated from b, or c 

 from d, is the reducing division demanded by Weismann's hypoth- 

 esis. The observations of Riickert and Hacker prove that the 

 transverse division is accompHshed during the formation of the 

 second polar body. 



2. Detailed Evidence 



We may now consider some of the evidence in detail, though 

 the limits of this work will only allow the consideration of some 

 of the best known cases. We may first examine the case of Asearis, 

 on which the first account is based. In the first of his classical 

 cell-studies Boveri showed that each tetrad appears in the ger- 

 minal vesicle in the form of four parallel rods, each consisting of 

 a row of chromatin-granules (Fig. 89, A-C). He believed these rods 

 to arise by the double longitudinal splitting of a single primary chro- 

 matin-rod, each cleavage being a 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 sup- 

 ported this view by further observations in 1890 on the polar bodies 

 of Sagitta and several gasteropods, in which he again determined, as 

 he believed, that the tetrads arose by double longitudinal splitting. 

 An essentially similar view of the tetrads was taken by Hertwig in 

 1890, in the spermatogenesis of Asearis, though he could not support 

 this conclusion by very convincing evidence. In 1893, finally, Brauer 

 made a most thorough and apparently exhaustive study of their origin 

 in the spermatogenesis of Asearis, which seemed to leave no doubt of 

 the correctness of Boveri's result. Every step in the origin of the 

 tetrads from the reticulum of the resting spermatocytes was traced 

 with the most painstaking care. The first step observed was a double 

 splitting of the chromatin-threads in the reticulum, caused by a divi- 

 sion of the chromatin-granules into four parts (Fig. 92, A). From 

 the reticulum arises a continuous spireme-thread, which from its first 

 appearance is split into four longitudinal parts, and ultimately breaks 

 in two to form the two tetrads characteristic of the species. These 

 have at first the same rod-like form as those of the germinal vesicle. 

 Later they shorten to form compact groups, each consisting of four 

 spherical chromosomes. Brauer's figures are very convincing, and, 

 if correct, seem to leave no doubt that the tetrads here arise by a 

 double longitudinal splitting of the spireme-thread, initiated even in 

 the reticular stage before a connected thread has been formed. If 



