LINVILLE: PULMONATE GASTEROPODS. 219 



fluid. In sections of the oldest uterine eggs from Limax maximus (first 

 polar cell completely formed) the head of the spermatozoon — still a 

 single, oval, homogeneous, and slightly swollen body — lies near the 

 centre of the eg<^, its long axis being directed more or less definitely 

 toward tha egg-aster. 



4. Eggs of LiMNiEA. 



All the material of Limnsea elodes was obtained from eggs already 

 laid. In the earliest stages thus secured, the first maturation spindle 

 had begun to move from the centre of the egg. In only one instance 

 (Plate 1, Figure 2) have I seen distinct remnants of the germinative 

 vesicle. 



II. Centrosome and Centrosphere. 



Recent investigation of the nature of the centrosome and the centro- 

 sphere has thrown considerable light on the variable nature of these 

 structures. The centrosphere cannot at present be considered a perma- 

 nent structure, but merely a temporary manifestation of an unknown 

 force. As long as the centrosphere appeared to be the region of the be- 

 ginning of the astral rays, there was good reason for assigning to it a 

 considerable degree of importance. In the light of the recent work on 

 Physa by Kostanecki und Wierzejski ( '96), and on Ascaris by Kos- 

 tanecki und Siedlecki ( '93), however, the centrosphere becomes of less 

 significance. They find that the astral rays extend into the centrosphere 

 and even to the centrosome itself. The centrosphere, according to these 

 authors, with whom I agree, is formed merely by a thjckening of the 

 rays. Wilson ('96, p. 234) calls attention to the concentric rings of 

 microsomes on the astral rays in the spermatogonium of Salamandra as 

 seen by Drtiner, and says that the innermost two rings, being especially 

 prominent, mark off a centrosphere composed of a medullary and a cor- 

 tical zone. Another case in point which Wilson discusses is that of the 

 rings of microsomes found by Heidenhain in leucocyte asters. The con- 

 dition shown in leucocytes, with the astral rays beginning at the centro- 

 some (the ultimate structure at the centre), tallies well with what 

 Kostanecki und Wierzejski found in the egg of Physa. If the centro- 

 sphere is to be taken out of the category of the permanent organs of the 

 CPill, it nevertheless represents a condition in the chemical and physical 

 phases of the cell which is worthy of further investigation. 



In the introduction to this paper, I have referred to the variable size 

 and condition of the centrosome. This is so intimately connected with 



