W. G. VanName — Embryology of Eustylochus. 277 



brane visible, but the polar bodies lie free in the space under the 

 shell. 



Eggs which lie in the uterus invariably contain a spermatozoon, 

 which lies fully as far removed from the surface as at the end of the 

 second maturation spindle (Figs. 5 and 36), It stains very deeply 

 throughout its whole length and lies curved and twisted into various 

 loops and bends. It is not apparent that it enters at any particular 

 part of the egg's surface or has at this stage any particular position 

 in relation to the spindle, which it often approaches very closely, 

 especially in Flanocera nebulosa. 



At this stage, especially in eggs of JEustylochus (it is not usually 

 conspicuous in Planocera), the egg-cytoplasm, immediately about 

 one end of the spermatozoon, has a more densely granular appear- 

 ance and stains more deeply (Figs. 5 and 6). In some preparations 

 this is exceedingly conspicuous (Fig. 6). I have not discovered 

 anything of the nature of a centrosome in this and attribute' it to 

 the disintegration of some non-nuclear portion of the spermatozoon. 

 By the time the egg is laid it has disappeared. 



In poorly preserved specimens it is not uncommon to find spots 

 and discolorations of various kinds and shapes near the spermatozoon. 

 There is no constancy in their appearance and they do not occur in 

 the best preparations. The Qgg can remain in the uterus in the 

 conditions shown in Figs. 5 and 36 for an indefinite period. No 

 further change appears to take place until it is laid. 



Later Stages of the First Polar Spindle. 



A section of a recently laid egg is shown in Fig. 7. The spindle 

 as in other polyclad eggs has a central position, and measures 

 between the centers of the asters three-eighths or more of the 

 diameter of the egg. The two centrosomes in each aster often lie 

 so near together that unless the extraction of the stain is carried 

 sufiflciently far they cannot be distinguished as separate, though 

 usually the centrosome, if but one appears, has an elongated form 

 showing that it may be double. The centrosphere surrounding them 

 appears homogeneous in structure, and is elongated in the direction 

 of the axis of the spindle. It has gradually become bounded by a 

 distinct and smooth outline, from which the rays, still rather few in 

 number and about one-half the length of the spindle, have their 

 origin. They branch and are lost in the spaces between the yolk 

 globules, which are quite uniformly distributed in the egg, and come 

 close to the spindle and centrospheres. 



