ORIGIN OF THE TETRADS 



255 



arose by double longitudinal splitting. An essentially similar view 

 of the tetrads was taken by Hertwig in 1890, in the spermatogenesis 

 of Ascaris, though he could not support this conclusion by very con- 

 vincing evidence. In 1893, finally, Brauer made a most thorough 

 and apparently exhaustive study of their origin in the spermatogene- 

 sis of Ascaris, which seemed to leave no doubt of the correctness of 

 Boveri's result. Every step in the origin of the tetrads from the retic- 

 ulum of the resting spermatocytes was traced with the most pains- 

 taking care. In the early prophases of the first division the nuclear 

 reticulum breaks up more or less completely into granules, which 



Fig. 127. Maturation-divisions in an insect, Anasa. [PAULMIER.] 



A. Primary spermatocyte in metaphase. B. Equatorial plate, showing ten large tetrads and 

 one small one; "odd chromosome" at o. C. Separation of the dyads. D. Telophase, which is 

 also a prophase of the second division. E. Secondary spermatocyte; division of the dyads; 

 small dyad shown undivided. F. Final anaphase ; small dyad near the lower chromosome-group. 

 (The figures are numbered from left to right. For later states, see Fig. 82.) 



become in part aggregated in a mass at one side of the nucleus 

 ("synapsis," p. 276), from which delicate threads extend through the 

 remaining nuclear space (Fig. 120, A). Even at this period the 

 .granules of the threads are divided into four parts. As the process 

 proceeds the chromatin resolves itself into a single spireme-thread, 

 consisting of four parallel rows of granules, which break in two to 

 form the two tetrads (var. bivalens), or is directly converted into a 

 single tetrad (var. nnivalens) (Fig. 120). From these observations 

 Brauer concludes that each tetrad arises from a rod, doubly split 

 lengthwise by a process initiated at a very early period through the 



