THE SPERMATOCYTE DIVISIONS OF THE ACRIDIDjE. 



BY C. E. M'CLUNG. 

 With Plates xv, xvi, xvii. 



/^WING to the importance of the question with regard tothemech- 

 ^-^ anism of heredity, the manner in which the chromatin is sepa- 

 rated in the two rapidly succeeding spermatocyte divisions has 

 attracted much attention. In the consideration of this problem, 

 materials derived from Ascaris megalocephala, diiferent species of 

 Copepods and several genera of insects have been largely used. The 

 results thus obtained are of a widely difTerent character; indeed, all 

 the possibilities of the jDrocess appear to have been exhausted in the 

 explanations of different observers. 



Fundamentally, the question involved might be stated somewhat 

 in this form : Given a rod ; derive from it by direct division four ap- 

 proximately equal rods. Obviously this might be accomplished by 

 splitting the rod twice longitudinally in planes at right angles to each 

 other, by cutting it across in three places, or by sjolitting it once lon- 

 gitudinally and tlien halving each of the resulting pieces by a 

 cross-division. 



The problem of securing from the spireme thread of the sper- 

 matocytes the quadripartite chromosomes which enter into the 

 subsequent divisions has, in fact, thus been solved by different in- 

 vestigators. The work of these scientists is now so well known that 

 it is hardly necessary to review it here, but in order to make a con- 

 sideration of the c^uestion more convenient, I will briefly advert to 

 typical cases. 



The simplest method, that of two longitudinal divisions, finds its 

 exponent in Brauer, who reached his conclusions from a study of the 

 spermatogenesis of Ascaris megalocephala. In this animal, it is as- 

 serted that the spireme thread divides by cleavage at right angles to 

 its length, into a number of segments equal to that of the subsequent 

 chromosomes. Each of these segments, then, by two divisions par- 

 allel to the length of the original thread, sjjlits into four rods, which 

 remain bound together by linin fibers. The two divisions of the 

 [spermatocytes then distribute these to the four resulting spermatids. 

 This conclusion, although apparently well substantiaied, remains an 

 dmost isolated exainple, and, owing to the fact that the composition 

 jf the chromosomes in the sexual cells of this worm is so anomalous, it 



[73]-K.U.Qr.-A ix 1-Jan. '00. 



