332 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the individuals ; for I have concluded from a study of the cells of the 

 two types in S. heros that the difference in size is due to differences in 

 nutrition. 



The testes were fixed in Gilson's fluid, embedded in paraffine, and 

 reduced to sections 3 to 5 micra thick. These were stained in Heiden- 

 haiu's iron-haeinatoxylin (some were counterstained with Congo red), 

 and studied under a magnification of from 1200 to 1800 diameters. 



The spermatogonia of S. subspiuipes in the resting stage are similar 

 in all respects to those of S. heros. They are small cells containing an 

 oval nucleus, in which no indication of chromatin can be observed ex- 

 cept the large, deeply staining nucleolus-like body, the karyosphere. 

 During the prophase small granular aggregations of chromatin appear 

 (Figure 1), and at the same time the karyosphere becomes so altered as 

 to show conclusively that, as in the sister species, the chromosomes are 

 derived from its substance. 



After the formation of the chromosomes from the karyosphere in 

 S. heros nothing of this body remains except the accessory chromosome. 

 In the present species, however, the residue is different. It consists of 

 two parts, a small, deeply staining chromatic body, the accessory chromo- 

 some, and a larger, less dense part, the plasmosome (Figure 1). The 

 latter part does not retain the haemotoxylin, but is nearly colorless, or 

 assumes a reddish-brown tint, when the haematoxylin is followed by 

 Congo red. During the later part of the prophase the plasmosome is 

 dissolved, and in the metaphase and later stages of mitosis no indications 

 of such material are to be found (Figure 2). 



The phenomena occurring during the later stages of the last sperma- 

 togonium seem to be identical in all essential respects to those already 

 described as existing in S. heros. At the end of the anaphase the 

 chromosomes are massed together so closely at opposite poles of the 

 elongated cell as to make their outlines indistinguishable (Figure 2). 

 Later, during the telophase, these chromosomes still aggregated into a 

 dense mass, lengthen out into slender granular segments (Figure 3). A 

 single element, however, the accessory chromosome, does not assume 

 this granular condition, but retains its homogeneous appearance and is 

 on this account very conspicuous (Figure 3). 



Upon the reconstruction of. the nuclear membrane the segments of 

 chromatin become distributed throughout the nuclear space (Figures 4,5), 

 causing the cells at this stage to bear a close resemblance to the sperm 

 cells of insects in the spireme condition. Here, however, the chromatin 

 is not a continuous thread, as it is said to be in the cells of many animals, 



