2 I O B YRNES. [Vol. XVI . 



vesicular. Although the nucleus increases in size, it retains its 

 position with reference to the place of formation of the polar 

 globules, being frequently held fast to the egg-membrane by 

 a " Zwischenkorper " (PI. XII, Fig. 27), which often persists 

 until both nuclei have reached their maximum growth. The 

 persistence of the Zwischenkorper is of some importance in 

 the later stages, being often the only means of distinguishing 

 the egg-nucleus from the nucleus of the spermatozoon. 



During the formation and growth of the egg-nucleus the 

 astrosphaere undergoes a series of very remarkable changes, 

 which may be referred to three distinct periods. These periods 

 are characterized by the formation, the growth, and the dis- 

 appearance of the spiral astrosphaere. From the time of the 

 extrusion of the second polar globule until the astrosphaere 

 begins to enlarge, the centrosphere retains unchanged the char- 

 acter of the centrospheres of the second maturation spindle 

 (PI. XII, Fig. 22). The astral rays are at first straight, but 

 while the aster is still small they begin to bend spirally. When 

 seen from the upper pole of the egg, the rays of the spiral 

 aster are bent toward the right in the direction of the move- 

 ment of the hands of a clock. Kostanecki and Wierzejski 

 have figured an aster in the egg of Physa, which shows the rays 

 at the inner pole of the second maturation spindle arranged 

 spirally ; the time of the appearance of the spiral aster in 

 Physa corresponds exactly, therefore, to the time of the forma- 

 tion of the spiral aster in Limax. 



During the second period, the period of growth of the spiral 

 aster, the centrosphere again becomes arranged in distinct zones 

 (PI. XII, Fig. 25). During this period the center of the aster 

 shows a deeply staining central body, surrounded by a clear 

 zone ("heller Hof "), which is traversed by a loose reticulum. 

 This reticulum connects the central body with the periphery of 

 the sphaere from which the astral rays diverge, and seems to be 

 formed by prolongations of the inner ends of the astral rays. 

 Sometimes an extremely minute granule (the " centriole " of 

 Boveri .-') can be seen at the center of the homogeneous central 

 body (PI. XII, Fig. 25). This deeply staining central body is 

 probably homologous to the deeply staining mass of granules 



