302 bulletin: museum of compakative zoology. 



1. The Spermatogonia. 



The spermatogonia, which, in young gonads, form a layer next the 

 mesogloea four or five cells deep, are at first hardly distinguishable from 

 ordinary ectodern cells ; but after a period of multiplication, resulting in 

 an undetermined number of cell generations, they enter upon a period 

 without multiplication, from which they emerge much increased in bulk. 

 The ordinary ectoderm cells measure about 5.5 /x in diameter and their 

 nuclei 3 p., while the largest spermatogonia and their nuclei are respec- 

 tively 10.5 \l and 8.5 /x in diameter. The amount of growth is, however, 

 very variable (Plate 2, Figs. 15 and 18), depending, it seems, chiefly on 

 the position of the cells in the gonad and the readiness of their access 

 to a supply of food. That the different sized cells are existing under 

 different metabolic conditions is shown by the condition of the cyto- 

 plasm, which contains in the larger cells many amorphous and non-vital 

 structures, which are seldom or never found in the smaller. The larger 

 cells in general appearance recall, in a striking way, the oogonia and 

 early oocytes. When closely packed together their outlines are roughly 

 polygonal, but when lying in more isolated positions, they readily as- 

 sume a more oval outline, showing that variability of form which is 

 so characteristic of coelenterate cells. The nucleus (Fig. 15), which has 

 more than half the volume of the cell, is spheroidal, bounded by a well- 

 defined membrane, and encloses one, or sometimes two, large nucleoli. 

 The remainder of its area is filled with a rather dense and distinctly 

 granular karyoplasm, through which is to be traced an irregular and 

 exceedingly delicate linin reticulum, bearing at its nodes distinct 

 karyosomes. Many of the strands of the net extend outward from the 

 substance of the nucleolus (Figs. 15, 17), which is very large. After 

 treatment with iron haematoxylin it sometimes appears, as in Figure 16, 

 densely stained and homogeneous, but in the majority of cases (Figs. 15, 

 17, 18) it shows a pale, even transparent, central area surrounded by a 

 deeply stained " shell." Occasionally, perhaps as the result of faulty 

 manipulation, this central region becomes highly refractive and sharply 

 outlined, but I am convinced that such appearances are artificial. It is, 

 of course, evident that we are dealing with a structure of the same 

 nature as that already discussed in the case of the endoderm cells ; that 

 is, here also the nucleolus is of compound nature, consisting of a cen- 

 tral sphere or plasmosome, surrounded by a shell, which is shown by 

 later changes to be of chromatic nature. This condition of the nucleolus, 

 while realized in somatic cells and oogonia as well as in spermatogonia, 



