2O8 MAULSBY W. BLACKMAN. 



cisely what should be expected. Each of the other chromosomes 

 is derived by the fusion of two of the spermatogonial chromo- 

 somes during the telophase of the last mitosis of the division 

 period. On the other hand, the accessory chromosome is de- 

 scended directly from a single element of the spermatogonium. 

 This being true, it is but logical to expect it to behave differently. 

 The primary object of the spermatocyte period is the reduction 

 of the chromosomes to one half the somatic number. It is 

 usually, if not invariably, the case, in arthropods at least, that 

 this period is characterized by two divisions of the chromosomes, 

 a longitudinal and a cross division. It is generally assumed 

 that, by one of these divisions the transverse division reduc- 

 tion is accomplished by the pulling apart of the chromosomes 

 at the point at which they were united in the preceding synap- 

 sis. Now as the accessory chromosome is not obtained by the 

 union of two spermatogonial chromosomes, this reducing divi- 

 sion is not necessary and does not take place. For these reasons 

 while the ordinary chromosomes are each composed of four 

 parts, /. e., are tetrads, this modified chromosome is made up of 

 but two parts, i. c., is a dyad. Furthermore, it is logically to be 

 expected that the accessory chromosome being dyad in its nature 

 would take part in only one of the succeeding divisions. This 

 peculiarity has indeed been observed by many investigators of 

 insect spermatogenesis and several explanations more or less 

 supported by observed facts, are offered in explanation thereof. 



In Scolopendra, as in other arthropods, the longitudinal 

 division of the chromosomes occurs in the first spermatocyte 

 mitosis. Strong indications of the character of this cleavage may 

 be seen in the metaphase of the first spermatocyte. Fig. 14, i 

 represents a typical chromosome at the time of the formation of 

 the first maturation spindle. At g is shown a tetrad of the same 

 kind undergoing metakinesis. By a comparison of these two 

 chromosomes it becomes evident that it is a longitudinal division 

 of the element which occurs. The mantle fibers are attached to 

 the two ends, and when the force which separates the halves of 

 the two chromosomes is applied, the two parts glide over each 

 other and seem to separate with the greatest reluctance. The 

 strongest proof that we are here dealing with an equation division, 



