340 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tached, leaving the nucleolus plainly visible, the conditions for observing 

 them are favorable. At this time there are usually three or four of these 

 vacuoles, their diameters being about one sixth to one tenth that of the 

 plasmosome (Figures 14—18). They are clear, transparent, unstained 

 spherules bounded by a definite outline, but enclosed in no distinguishable 

 membrane. Later, as the prophase advances and the tetrads take on 

 their characteristic form (Figures 19, 20), these vacuoles increase both in 

 size and number, until, just before the nuclear membrane breaks down, 

 they are often so numerous that the nucleolus is a spongy mass bearing 

 little resemblance to its previous appearance. During this time the 

 plasmosome has not appreciably altered in size ; however, in the very 

 late prophase, only a little of the original material remains. 



This, I believe, might be explained on the supposition that the vacuoles 

 represent material — probably hyaloplasm — obtained from without, 

 while the nucleolus at the same time gives off material to the nucleus. 

 The material thus gained by the nucleus may represent merely a by- 

 product of cell metabolism, but the fact that the nucleolus degenerates 

 immediately preceding cell division might be taken to indicate a more 

 important function. Three explanations concerning the purpose and fate 

 of the nucleolus suggest themselves : (1) the material may be used for 

 the nutrition of the cell in general, (2) it may be used directly in the 

 formation of the spindle fibres, or (3) it may contribute to the achromatic 

 portion of the chromosomes. I believe that the first of these alternatives 

 is realized in the case of S. subspinipes. In S. heros, where the archo- 

 plasmic structures are very well developed, no nucleolus is to be found 

 at any stage of the spermatocyte. It further seems very often to be true 

 that where the proportion of chromatin in the cell is especially large, as 

 in the spermatocytes of insects and amphibians, no nucleolus whatever 

 is present, thus pointing to the independence of the chromosomes. On 

 the other hand, in cells in which there is a great preponderance of 

 cytoplasm, as in egg cells, nucleolar structures are of very general 

 occurrence. 



The behavior of the chromosomes during the two following mitoses is 

 so similar to that already described in S. heros that it hardly requires 

 separate description. The result of the two spermatocyte divisions is 

 four spermatids from each primary spermatocyte. Of these four, two 

 contain one more chromosome each than do the other two, just as is the 

 case in S. heros. The accessory chromosome is not divided in the first 

 mutosis, but goes entire to one cell. In the second division, when the 

 other chromosomes are uuderfroiu" a reduction division, this divides longi- 



