M'CLUNG: CHROMOSOMES OF ANASA TRISTIS. 379 



ing and a broken accessory chromosome midway between them. 

 In all of these cells the chromosomes are thin and of indefinite 

 outline and have apparently suffered much in the process of 

 preparation. There is not a clear case of a divided accessory 

 chromosome in the series, although individual cells, doubtless the 

 best that could be found, are selected and photographed. That 

 later stages show conclusively the undivided accessory chromo- 

 some, not alone in selected cases but in every cell in a micro- 

 scopical field, is demonstrated in plate LXXI, figure 15, of this 

 paper. 



The objection may be raised that such a stage does not show 

 all the chromosomes; but it is not a valid objection because it 

 is impossible to photograph all the chromosomes at this stage 

 on account of their apparent fusion into a mass. Moreover, 

 the essential facts are admitted by all. It is agreed th.nt all 

 the chromosomes, with the exception of the accessory chromo- 

 some, divide so that each group contains ten ordinary chromo- 

 somes; it is agreed that the lagging chromosome is the acces- 

 sory chromosome. The question therefore to be answered 

 concerns the behavior of the lagging chromosome. Are its 

 derivatives found in each of the separated daughter cells of the 

 second spermatocyte; or is it, undivided, included in only one 

 of the two ? The evidence on both sides has been presented in 

 statement, drawing, and photograph and the judgment of those 

 interested must, in the absence of personal study, be based 

 upon the presentations. The crux of the situation lies here, 

 for if the accessory chromosome divides there is no dimorphism 

 of the spermatozoa and no visible sexual differentiation of the 

 paternal germ cells, but if it remains undivided then there is a 

 dimorphism of the spermatozoa and a consequent visible chro- 

 mosome differential between two numerically equivalent 

 classes. 



It is pertinent to consider also whether in evaluating the 

 evidence in this case, any considerable number of genuine 

 instances of unusual conditions can be held to invalidate the 

 common conclusions reached by numerous independent stu- 

 dents of these phenomena. There can be no question of the 

 occurrence of abnormal conditions in the development of the 

 germ cells. Entire cysts, or even testes, become degenerate 

 from imperfect coordination in development ; certain daughter 



?-Univ. Sci. Bull., Vol. V. No. 20. 



