No. I.] THE EGG OF UMAX AGRESTIS. 203 



the archiamphiaster stage the astrosphaeres of the egg of 

 Limax agrestis conform very closely to the type of astro- 

 sphaere described by Van Beneden and by Boveri for 

 Ascaris. 



The centrosphere has a distinct peripheral zone, the " cor- 

 tical " zone of Van Beneden, or "archoplasm" of Boveri, 

 which stains intensely black with iron-haematoxylin. When 

 stained with Lyons blue and borax-carmine it becomes in- 

 tensely blue. This outer zone is often very sharply marked 

 off from the rest of the cytoplasm, although the inner ends of 

 the astral rays are in direct contact with its periphery (PI. XI, 

 Figs. 2, 3, and 6). On its inner circumference the outer 

 " cortical " zone of the centrosphere is marked off with equal 

 distinctness from a perfectly clear structureless zone, the 

 "medullary" zone of Van Beneden. Sometimes, however, 

 the concentric layers of the centrosphere are less sharply 

 outlined than in Fig. 2, appearing as in Figs. 4 and 5. Figs. 

 4 and 5 represent sections through an ^^g that was killed 

 immediately after it was laid. The ^gg is of approximately 

 the same age as that represented in Fig. 6. The centro- 

 sphere in Figs. 4 and 5 is composed of four concentric rings, 

 which are alternately dark and light, as in the centrospheres 

 formed by only two rings. As the number of rings increases, 

 the boundaries between them become less sharply defined. 

 Here, as in PI. XI, Figs. 2 and 3, the astral rays diverge from 

 the periphery of the outermost dark ring or zone. 



Within the clear inner zone of the centrosphere there is 

 a deeply staining central mass, consisting of several granules 

 that form the centrosome or centrosomes. These granules are 

 often grouped irregularly, as in Fig. 6, but sometimes they are 

 arranged so as to appear like two short thick rods in the form 

 of dumb-bells (PI. XI, Figs. 2 and 5). In haematoxylin the cen- 

 trosomes stain almost black ; in Lyons blue and borax-carmine 

 they stain blue. The centers of the astrosphaeres in the archi- 

 amphiaster sometimes fail to show deeply staining central gran- 

 ules, but this is probably due either to the method of preser- 

 vation or to some accident in staining, since the centrosomes 

 are usually present even at a much earlier period. The inner 



