The formation of the second maturation spindle in the eggs of Succinea, Physa, and Planorbis.

IN a previous communication (Raven, Escher, Herrebout, & Leussink, 1958) the maturation of the egg was described 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 A. reticulatus and Limax flavus this centrosphere contains a pair of centrioles which move apart to opposite poles of the developing spindle. This is symmetrical and develops an aster at both ends, arising in the cytoplasm outside the original centrosphere. In Limnaea stagnalis the centrosphere of the first maturation amphiaster contains only one centriole, which remains undivided. When the centrosphere elongates, the centriole moves as a whole to its outer pole. The spindle developing from the centrosphere is therefore asymmetrical and egg-shaped, and aster formation occurs at its pointed outer end only. The inner aster of the second maturation spindle is provided by the sperm aster, which fuses secondarily with the blunt inner end of the spindle. In discussing this aberrant course of the second maturation division in L. stagnalis, it was concluded that its primary cause is the failure of the egg cytocentre to divide properly. This causes the lack of astral radiations at the deep end of the developing second maturation spindle, and enables the sperm aster to fuse with this end of the spindle. It was argued that a mutation bringing about a precocious inactivation of the egg cytocentre, in conjunction with the circumstance that a sperm aster, free to take over the task of the failing deep maturation aster, is present at the right moment, might have led to this course of events. As it appeared conceivable that a modification of development occurring at such an early stage might have a profound evolutionary significance, it seemed important to investigate the taxonomic distribution of this peculiar mode of formation of the second maturation spindle.