DESCRIPTION OF PLATES 



All figures were made with the camera, at stage level, with Zeiss apochromatic 

 2 mm. oil immersion objective; figures 1 to 6, 24 and 27 with no. 4 compensating 

 ocular, all the others with no. 18. All sections from eggs, preserved in Zenker- 

 formol, except figure 27 drawn from a section of an egg, preserved in Zenker. 

 For a detailed description of the figures see the text. 



PLATE 1 



EXPLANATION OF FIGURES 



1 Immature sea-urchin egg with a large germinal vesicle, containing a deeply 

 stained nucleus, chromatic granules and filaments. The cytoplasm shows the 

 presence of large, dense ribbons, similar to the large plastosomes of the sperma- 

 togonia of insects. 



2 Mature sea-urchin egg with a small achromatic female pronucleus; the 

 cytoplasm contains large chromatic bodies. 



3 Egg 40 minutes after fecundation. Single cytoplasmic radiation around 

 the oval segmentation nucleus; small chromatic granules in the cytoplasm; de- 

 velopment of the chromatin in the nucleus in the immediate vicinity of the cyto- 

 plasm. 



4 Parthenogenetic egg after remaining 40 minutes in sea-water. Single 

 cytoplasmic radiation around the spherical nucleus and accumulation of chro- 

 matic granules; development of the chromatin in the nucleus and its connection 

 with the outside laying chromatic material. 



5 Parthenogenetic egg after remaining 40 minutes in sea-water. Many astro- 

 spheres and fragmented nucleus. 



6 Parthenogenetic egg after remaining 1 hour in sea-water; single radiation 

 around the oval nucleus. Chromosomes (drawn from two sections) defectively 

 developed in the nucleus; many small chromatic granules, lying in the cytoplasm. 



594 



