NUCLEOLI IN PEDICELLINA AMERICANA. 



349 



floor of the atrium and there multiply very rapidly. This in- 

 crease is much more pronounced in the male than in the female 

 and gives the first and principal basis for the distinction now 

 present between the sexes. The bi-partite testis, with its sperma- 

 togonia, has grown considerably and the latter are mostly in the 

 resting condition. The nuclei are large, being surrounded by a 

 very thin layer of cytoplasm. Within the nucleus, the chroma- 

 tin, in the form of a reticulum, stains very lightly ; indeed, in 

 well extracted iron-haematoxylin preparations shows scarcely at 

 all. But what is more important is the almost invariable presence 

 of the one or the two nucleoli. These stain uniformly and in- 

 tensely black with hasmotoxylin and retain their color long after 

 that of the other cell elements has been extracted. When in 

 pairs, they are placed at opposite points of the nucleus ; when 

 single, the nucleolus is found more nearly in the middle. The 

 cells of the other tissues have preserved the same condition of 

 the nuclei as was above described for the younger polyp. 



In older testes, many more cells are present (Fig. 2), and these 

 are proved, by the unreduced number of the chromosomes, to be 

 also spermatogonia. Among these are cells of many sizes both 



FIG. 3. 



at rest and in mitosis from which it is certain that there are several 

 generations of spermatogonia. In tracing the development of the 

 spermatogonium from the telophase of the preceding division, 

 attention is attracted by the appearance of the two nucleoli. At 

 the beginning of the reconstruction there is as yet no trace of 

 them. This stage is soon followed by one where the nuclear 

 membrane has reappeared, the chromatin being almost entirely 

 unaffected by haematoxylin. At two opposite points of the 

 nucleus there now appear the two nucleoli as yet very small but 



