STUDIES ON CHROMOSOMES. 243 



should imitate so consistently the behavior of an X-element. 

 It seems to me that one must be credulous indeed to catalog it 

 all as accidental. 



When I take into account all of the evidence which I have been 

 able to accumulate through some ten years of study on this prob- 

 lem, I feel more firmly convinced than ever that in this distinc- 

 tive curved chromosome we are dealing with a body comparable 

 to the so-called X-element of other forms. Certainly the evi- 

 dence of this, as regards numbers of X-like chromosomes actually 

 seen is much stronger than that which has been accepted un- 

 questioned for certain of the less typical cases in invertebrates. 

 However, in view of the condition which I have found existing 

 in certain somatic and early germ-cells of the male where in a 

 large number of instances two special curved chromosomes 

 occur, I am strongly of the opinion that the large curved element 

 of the primary spermatocyte is in reality these two earlier ele- 

 ments fused into one. It is just about the size that two such 

 elements would have when fused; moreover, no further trace of 

 the latter is discernible after the spermatogonial stages are 

 passed. Then again, such a doubling would account for the 

 relatively larger size of the curved element in comparison to 

 the other chromosomes of the primary spermatocyte. My inter- 

 pretation of the condition then, insofar as I can analyze the 

 probabilities of the case, is that the two special curved elements 

 of the spermatogonia fuse in the primary spermatocyte and 

 act as a single element, typically passing undivided to one pole 

 of the mitotic spindle and thus producing a dimorphic condition 

 of the ensuing spermatocytes of the second order, half of which 

 will contain the X-like element, half be without it. 



SECONDARY SPERMATOCYTES. 



The secondary spermatocytes are of approximately the same 

 size as the original spermatogonia. Although resting nuclei are 

 not infrequently to be found, in many cases the chromosomes of 

 the primary spermatocyte telophase rearrange directly to form 

 the metaphase of the secondary spermatocyte. In so doing 

 there is a marked tendency for the chromosomes to fuse by 

 twos so that instead of the expected sets of eight and nine re- 



