308 C. E. MCCLUNG. 



instead of univalent. The chromatids composing this multiple 

 chromosome l are amongst the largest in the cell and are easily 

 distinguishable. During the anaphase it is seen that the multiple 

 chromosome exhibits the so-called heterotypical form of division, 

 and the figure produced is exactly the same as that resulting from 

 the division of the tetrads in the first spermatocyte. In reality we 

 have here a tetrad that is in every respect the same as those in 

 the succeeding generation. A precocious synapsis of chromo- 

 somes in the spermatogonia is responsible for the unusual type 

 of division in this generation of cells. 



There can be no mistake about the nature 

 f thi 8 multiple chromosome, for the line of 

 fusion between the two chromosomes is 

 clearly marked, and the mantle fibers of the 

 FIG. 2. The hexad metaphase attach at the center of the U- 

 multiple chromosome of shaped element. In the anaphase the con- 

 Hesperotetti* in the first tragt between this chromosome and the 



spermatocyte prophase. 



The accessory chromo- others is very marked, for the daughter 

 some "a.c." is homo- chromosomes of the latter ascend to the 

 geneousand the ordinary poles as stra j g ht rods with the fibers at the 



chromosomes granular, .. 



ends, while the former are U-shaped and 

 as is usually the case for 



these elements at this have the fibers annectent at the center, or 

 stage. where the chromatids are joined .together. 



The early prophase of the first spermatocyte shows this 

 bivalent element among the others, which have by this time also 

 become bivalent, but it may still be distinguished by its greater 

 leneth. When the chromosomes commence to shorten and con- 



o 



dense the accessory chromosome attaches itself to one end of the 

 large bivalent element, thus forming a trivalent chromosome. 

 As thus constituted the multiple chromosome is much longer than 

 any of the others, and clearly shows the limits of the three parts. 

 A very peculiar condition prevails at this time. As is usually the 

 case in the prophase, the ordinary chromosomes are granular 

 and irregular in outline, while in striking contrast the accessory 

 end of the multiple chromosome is homogeneous with sharply 

 cut boundaries. The trivalent chromosome, or hexad, thus 



1 I shall use the term " multiple chromosome " for all elements containing more 

 than two chromatids. 



