2l8 KATHARINE FOOT AND E. C. STROBELL. 



accessory chromosome of other forms, and if this interpretation is 

 correct we may expect to find a large bivalent or two univalent 

 chromatin nucleoli in the growing oocytes. 



II. We find the chromatin nucleolus of the spermatocyte per- 

 sisting through the entire growth period, Photos I to 10. All 

 the resting spermatocytes shown in these photos, are from testes 

 which contain many first spermatocyte metaphases, in which 

 the idiochromosomes show the typical inequality in size. We 

 have selected preparations of the resting spermatocytes showing 

 variations in the relative size of the two chromatin nucleoli 

 (when present) in order to demonstrate their frequent lack of con- 

 formity to the size relations of the idiochromosomes. Photo 10 

 shows a nucleolus persisting until the chromosomes are formed 

 and we have several photographs demonstrating its presence to a 

 still later stage, where the seven bivalents can be counted, but we 

 shalLreserve the discussion of these later stages for a subsequent 

 paper. 1 



As the chromatin nucleoli of the first spermatocyte of EitscJiis- 

 tiis persist through the entire growth period, we certainly have a 

 right to expect to find in the egg the equivalent of the large 

 chromatin nucleolus during these same stages, if as asserted it 

 owes its origin to the egg. Again, if the so-called male sex- 

 determinant (the small chromatin nucleolus) persists through the 

 growth stages in the male cell, there seems to be no clear reason 

 why the so-called female sex-determinant (the larger chromatin 

 nucleolus) should not persist through the growth period of the 

 oocyte. 



A study of the oocytes shown in Photos 1 1 to 30 demonstrates 

 that in none of these cells can be found a structure resembling in 

 any way the chromatin nucleolus shown in the spermatocytes of 

 Photos I to 10. This is clearly demonstrated in the sperma- 

 tocytes and oocytes of Plate I. There is no structure in the 

 oocytes of Photos n to 19 which resembles in the least the 

 chromatin nucleolus of the spermatocytes of Photos i to 10. The 

 only nucleolus in the oocytes is a relatively .large achromatic 

 structure, first clearly demonstrated in the older oocytes. It is 



1 We have a number of photographs showing the transition stages between Photos 

 9 and lo, but lack of space prevents their reproduction on these plates. 



