The formation of the second maturation spindle in the eggs of Limnaea, Limax, and Agriolimax.

EGG maturation in Limnaea stagnalis has been described by Raven (1945, 1949). It exhibits some peculiarities, by which the Limnaea egg seemed to stand apart from all other animal eggs in which the maturation processes have been accurately studied so far. In the first place it appeared that the second maturation spindle arises by a direct and gradual transformation of the centrosphere at the inner end of the first maturation amphiaster, remaining in the egg after the extrusion of the first polar body. Secondly, the above-mentioned investigations revealed the still more astonishing fact that the inner aster of the second maturation spindle is none other than the sperm aster, which arises quite independently round about the time of the extrusion of the first polar body, grows considerably in size while the second maturation spindle is being formed, and then fuses secondarily with the latter's deep end. After the extrusion of the second polar body the aster becomes free again. It shifts to a more central position in the egg, where it remains visible for some time, but gradually disintegrates, and disappears long before the prophase of first cleavage. Although our previous investigations, executed on a sufficiently large number of eggs, admitted of no other interpretation of the facts, this course of events appeared to be so much at variance with all descriptions given in the literature on maturation in other species that further confirmation was felt desirable. It appeared probable that an accurate study of the maturation process in eggs of related species might contribute to a better evaluation of the events found in Limnaea. One could hardly expect that such a fundamental process as matura-