148 SMALLIVOOD. [VOL. II. 



follow the details very carefully. The spindle is developed 

 when the central corpuscles separate. At this time a very 

 faint line can be seen connecting the new corpuscles; as 

 the distance between them increases, the line becomes more 

 distinct, until a central spindle can be clearly distinguished. 

 In the mean time the line limiting the centrosome has become 

 broken into pieces, which gradually become smaller and smaller 

 until they cannot be distinguished from the granules of the 

 cytoplasm. While these changes have been taking place, this 

 broken line has served to mark the outer limit of the medullary 

 zone. The old medullary zone has disappeared, and between 



the central corpuscle and the broken wall of 

 the centrosome we have a new medullary 

 zone, which is the cortical zone of the 

 second polar spindle. The process is as 

 follows : as the walls of the centrosome begin 

 to break down, an area next to the central 

 corpuscles and at each end of the centro- 

 FIG. - Meuphase of some appears ; this begins to take a plasma 



first maturation spindle. . /I- \ -T-I 



TWO central corpuscles stain (Fig. 3). The area gradually surrounds 

 at each pole. Cortical ^ e ce ntral corpuscle and all of the space at 



zone limited by a dotted 



outline. The medullary thecudof the spindle between the central 



zone does not take a i i ., i nr^i 



plasma stain at this corpuscle and the wall of the centrosome. 

 stage, sperm head solid j n the mean time the central corpuscle has 



and elliptical. 



increased in size and is to become the cen- 

 trosome of the second polar spindle ; it ultimately becomes 

 differentiated into a central corpuscle and a medullary zone. 



Fig. 2 shows two central corpuscles in the centrosome at 

 each pole of the spindle. Linville 1 shows a similar but not 

 identical stage. The centrosome of the outer pole of the 

 spindle does not enlarge more than is shown in the figure ; as 

 it reaches the surface of the egg, it breaks down and the cen- 

 tral corpuscles form the division centers in the first polar 

 body. 



There is no telaphase in the first maturation spindle. The 

 nearest approach to such a stage is shown in Fig. 3, where the 



1 " Maturation and Fertilization in Pulmonate Gasteropods," Bull. Mies. Comp 

 Zoo!., I/arrant. Vol. xxxv, No. 8. 



