The Germinal Spot in Echinoderm Eggs. 



the question of the individuality of the chromosomes, nothing definite can be 

 adduced. The process of chromosome formation here seems to be unique 

 among echinoderms. The only similar case known to me is that described by 

 Wilson in some of the sets of sea-urchin eggs stimulated to develop partheno- 

 genetically by MgQ 2 , and here the chromatin nucleolus first resolved itself 

 into a chromatic reticulum which subsequently broke up into chromosomes. 



OPHIOCOMA PUM1LA. 



The eggs of this brittle-star are of moderate size, being about that of 

 Astcrias forbesii. Figure n shows an egg near the culmination of the 

 growth-period. Maturation is probably imminent, for the nucleus has 

 moved near the periphery and the nuclear wall is much shriveled. This 

 assumption is confirmed by the fact that male individuals of this same col- 

 lection carried sperm in ripe condition and very active. The cytoplasm is 

 of the reticular type, with many microsomes and innumerable large dark- 

 staining yolk-spherules. The nucleus has a homogeneous or very finely 

 granular structure and remains unstained in basic dyes. Eosin reveals a 

 delicate network. Scattered through the achromatic nucleoplasm are irregu- 



FIG. 8. Young ovum of Ophiocoma pumila; nucleolus homogeneous; intensely chromatic 

 and with definite contour; nuclear reticulum wide-meshed, heavy and very chromatic; 

 cytoplasm reticular, dark and with abundant granules. X 15- 



FIG. 9. Nucleus at stage near culmination of growth-period; nucleolus intensely chromatic; 

 reticulum massed at one pole and beginning to break up into chromosomes, some of 

 which have a tetrad form. The nucleoplasm appears homogeneous or finely granular 

 and is pale-staining. The cytoplasm is reticular with many large yolk-spherules. 

 X 1500. 



FIG. 10. Nucleus at slightly later stage; wall much shriveled. A portion of the chromatin 

 thread is still breaking up into chromosomes, some of which have the form of tetrads, 

 and frequently one or several are attached to the nucleolus. X 1500. 



lar, mossy, deeply-staining chromosomes. They vary much in size and 

 shape, but some appear as tetrad-like bodies (figs. 9-13). The nucleolus 

 has persisted as a compact, homogeneous, intensely chromatic structure with 

 sharp outline and of original bulk. It is partly surrounded by what in many 

 cases appears to be a vacuole. In figure n a delicate pale-staining reticu- 

 lum appears in the vacuole. In many eggs at this stage (or probably of a 

 slightly later stage) the nucleolus seems to disappear by lysis, leaving a pale 

 outline of its original form. I was at first inclined to interpret the apparent 



