The Germinal Spot in Echinodenn Eggs. 



1 1 



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. At a 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 forbesii, 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 13 



FIG. 12. Nucleus at culmination of growth-period showing chromosomes of various shapes 



and sizes scattered through a pale, homogeneous or finely-granular nucleoplasm. 



X 1500. 

 FIG. 13. Nucleus 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. n. 



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- 

 ponoe esculenta (except that Hipponoe 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 chromatm 

 nucleolus in addition to the plasmosome common to both forms. 



