ABSTRACT
Egg maturation has been studied in Agriolimax reticulatus, Limax flavus, and Limnaea stagnalis with special regard to the formation of the second maturation spindle.
In the three species the second maturation spindle arises by direct transformation from the deep centrosphere of the first maturation amphiaster.
In Agriolimax reticulatus and Limax flavus there is a pair of centrioles moving apart to opposite poles of the developing spindle. In Limnaea stagnalis there is only one centriole, which remains undivided and moves as a whole to the outer pole of the spindle.
The developing spindle in Agriolimax and Limax is symmetrical and gets an aster at both ends, arising in the cytoplasm outside the original centrosphere. The developing spindle of Limnaea stagnalis is asymmetrical, egg-shaped, and aster formation occurs at its pointed outer end only.
The inner aster of the second maturation spindle in Limnaea is provided by the sperm aster, which fuses secondarily with the blunt inner end of the spindle.
There are some indications that the fusion between these two structures is accompanied by a certain cytochemical activity in the region of fusion.
The sperm aster in Limax flavus appears some time after the extrusion of the first polar body; it does not fuse with the second maturation spindle but disintegrates by vacuolization soon after the extrusion of the second polar body. In Agriolimax reticulatus no sperm aster has been observed.
In the three species the dyads apply themselves closely against the centrosphere but do not penetrate into it. In Agriolimax and Limax they lie as a compact group on one side against the equatorial region of the developing spindle. In Limnaea they become arranged into an irregular ring, which encircles the outer pole of the developing spindle and gradually shifts to the equatorial region of the spindle. In the three species the dyads only penetrate into the spindle when the latter has been completely formed.
The results are discussed with reference to the relationships in the Pulmonates in general. It is concluded that the sperm aster is a more or less rudimentary structure in this group.
In Limnaea the sperm aster has taken on a new function in the mechanism of polar body extrusion.
The primary cause of the atypical development of the second maturation spindle in Limnaea is, presumably, the precocious inactivation of the egg cyto-centre, which divides no more after the completion of the first maturation division.
