ACCESSORY CHROMOSOME IN ANASA TRISTIS. 121 



It is only when the chromosomes or parts of chromosomes are 

 abnormal that they show a condensed chromatin mass, or masses, 

 suggesting a resemblance to a nucleolus. In many cases one of 

 the arms of a cross-shaped chromosome will resemble a round 

 dense nucleolus and this may appear in from one to five of the 

 crosses, and again both arms, or the entire cross may have de- 

 generated into a compact, deeply staining mass of chromatin. 

 We have a number of photographs of connecting stages between 

 these extremes, and they leave no doubt that the normal chro- 

 mosome resembles in no way a nucleolus. 



Our preparations also demonstrate that the " chromosome nu- 

 cleolus " like the plasmosome of the egg of Allolobopliora fcctida 

 has disappeared, as a rule, when the chromosomes are formed 

 (early prophase) very rarely persisting until after the chromosomes 

 have attained their definite shape. 



Our smear preparations further demonstrate the absence in the 

 resting spermatocyte of any other structure which can be inter- 

 preted as a nucleolus. We approached the study of this form with 

 the hope of being able to identify a structure in the male cell 

 which could be interpreted as the homologue of the " accessory 

 nucleolus " of the egg, 1 but we have found no structure suffi- 

 ciently pronounced or constant to justify our interpreting it as 

 an " accessory nucleolus." 



Paulmier ('99), Montgomery ('01) and Wilson ('o5-'o6) have 

 all indicated a second nucleolar-like structure in the resting sper- 

 matocyte which they interpret as the true plasmosome, but we 

 have been unable to demonstrate a second nuleolar-like structure 

 in our smear preparations. However in sections of testes fixed 

 with Hermann's fluid and stained with iron haematoxylin and with 

 anilin stains, we often find nuclei of resting spermatocytes in 

 which a second nucleolar-like structure is differentiated, but the 

 complete absence of such a feature in our smear preparations, 

 makes us hesitate to interpret these two structures as the homo- 

 logues of the plasmosome and accessory nucleolus of the egg. 



Again our preparations demonstrate that the so-called uni- 

 valent " heterotropic " chromosome is distinctly a bivalent. Its 

 constant bivalent character indicates that it represents in value 



'Foot & Strobell, 1905. 



