562 Theophilus s. Painter, 



4. Alwaysassociated with the accessory 4. Always separate from the accessory 



chromosome during the first chromosome during the first 



maturation division, and pass division may divide ; have spindle 



early with that body undivided fibers of their own. Often lag 



to one pole. behind in the spindle. 



5. Appear to divide in the second 5. Do not divide in the second division 



maturation division. but lag behind in the spindle. 



6. Ultimate distribution seems to 6. Ultimate distribution very irregular 



follow some rule. and apparently simply a matter 



of chance. 



In such forms as Maevia or Amaurohius, the differences between 

 the ctetosomes and planosomes are very striking, but the question 

 whether or not intermediate forms may not exist is not answered. 

 After a study of other forms, including species of eleven families of 

 spiders, all of the evidence would seem to indicate that the two 

 types of bodies are fundamentally different. 



Origin of ctetosomes and planosomes. 



Three possibilities will be considered for the origin of the 

 ctetosomes and planosomes. 1. That they arise from the Y-chromo- 

 some, as suggested by Wilson (Study VI). 2. That thej^ are micro- 

 chromosomes which have degenerated. 3. That they come from 

 autosomes which have lost their function, 



1. Wilson, in his study of Metapodius, suggests that the super- 

 numerary chromosomes of that form are derived from the Y-element, 

 whicli failed to separate from the X-element during the second 

 division. In the following generations the Y- element would not have 

 any function, as the X-element would predominate and thus the 

 former body would pass through the division as a "supernumerary 

 chromosome". Stp^vens (1912) was inclined to explain the super- 

 numeraries of Ceuthophilus and Diabrotica soror in the same way. 



Will sucli an explanation be satisfactory for tlie supernumeraries 

 of spiders? To begin with the case is somewhat dilierent, for in 

 the Hemiptera, in closely allied forms, the Y-element may be present 

 or absent. In the spiders, however, in the thhteen familes studied 

 no evidence of the Y-element was found. In this case, would it be 

 logical to derive the supernumerary chromosomes from it? But 

 there are still more striking reasons why the origin suggested by 

 Wilson will not apply to all of the supernumeraries of spiders. 



When we consider the planosomes, such an explanation proves 

 inadequate to account for the facts, 'i'lie valions works on spiders 



