The Germinal Spot in Echinoderm Eggs. 11 
maturation. The vacuole when present is to be interpreted as the result of 
a resorption of the plasmosome or perhaps plastin ground-substance. 
Ophiocoma presents a clear case where the chromosomes are derived 
exclusively from the nuclear reticulum. Ata very early stage in the growth- 
period the ovum has a coarse granulo-reticular cytoplasm (fig. 8). The 
germinal vesicle contains a homogeneous, intensely chromatic nucleolus of 
sharp contour. The nuclear reticulum is heavy, wide-meshed, and very 
chromatic. Later stages show the segregation of this reticulum at one pole 
- of the nucleus—sometimes about the nucleolus, sometimes at the opposite 
pole—as a tangled spireme. When insufficiently destained in the iron-alum 
solution, such stages do not reveal the thread-like character of this structure, 
but the entire area stains as a solid irregular chromatic mass. Subsequently 
the thread unravels and segments into chromosomes (fig. 9), some of which 
soon assume the shape of tetrads. They have an irregular outline and mossy 
appearance. Frequently one or several are attached to the chromatic nucle- 
olus, and here, as in Asterias forbesti, there seems to be a tendency on the 
part of the thread to become attached to the nucleolus. Figure 9 shows 
the spireme partially segmented into chromosomes and the remaining thread 
12 
Fic. 12.—Nucleus at culmination of growth-period showing chromosomes of various shapes 
and sizes scattered through a pale, homogeneous or finely-granular nucleoplasm. 
Fic. ela at final stage of growth showing chromosomes with appearance of tetrads 
and an intensely chromatic nucleolus connected with a larger spherical body of sharp 
contour and with a chromatic reticulum corresponding to the vacuole of fig. 11. 
X 1500. 
almost in contact with the nucleolus. In figures 10, 12, and 13 are shown 
later stages of the same process of chromosome formation, the process being 
not yet complete in figure 10. Figure 12 shows 18 chromosomes (reduced 
number) and figure 13 shows 17 chromosomes. The exact number could 
not be definitely determined, but it is somewhere close to 18. 
Here the chromosomes arise very clearly from the nuclear reticulum. 
The process here agrees essentially with that previously described for Hip- 
ponoé esculenta (except that Hipponoé showed no plasmosome), but differs 
again from what Wilson described for some of the two sets of Toxopneustes 
eggs artificially fertilized with MgCl, in that there is here a chromatin 
nucleolus in addition to the plasmosome common to both forms. 
