No. I.] POLYCHOERUS CAUDATUS. 157 



The capsule is soluble in weak hydrochloric and nitric acids. 



Sections through the animal containing mature and nearly 

 mature ova show that a small, inconspicuous polar globule 

 spindle is formed some time before the ovum is laid. In some 

 cases two polar globules may be seen after the ovum has been 

 laid, adhering to its surface on the macromeres A or A^ close to 

 the point where the second cleavage plane intercepts the first. 

 While the ovum is still within the parent, and shortly after the 

 polar globules have been formed, a remarkably large and dis- 

 tinct spindle is formed, which, since it appears directly after 

 the formation of the polar bodies, must be the first segmenta- 

 tion spindle. It may be readily seen by placing the animal 

 under a cover slip and by drawing off the excess of water with 

 filter paper, so that the cover slip exerts sufficient pressure 

 to cause the animal to fully expand, but at the same time does 

 not rupture it. In ova examined in this manner the nucleus 

 may appear as a round almost transparent spot of good size in 

 the center of the ovum ; in other ova it is somewhat elongated, 

 while in others it appears as a large dumb-bell-shaped structure 

 (Fig. 31) which occupies the greater part of the ovum. Indeed, 

 the elongation of the nucleus, the formation of the spindle, 

 and drawing apart of the polar suns, may be observed in one 

 specimen examined at intervals of a few hours. 



That spindles presenting this peculiar appearance are com- 

 mon in the ova of many Turbellarians is shown by von Graff 

 (2), who in his monograph figures individuals belonging to 

 three different genera, in all of which ova containing such 

 spindles are shown. He does not, however, discuss this matter 

 in the text. 



A very remarkable phenomenon occurs in connection with 

 this spindle ; for if the animal be kept too long under the 

 cover slip or placed under certain abnormal conditions, the 

 polar suns grow dimmer and draw closer together and the 

 dumb-bell-shaped structure disappears entirely until the nucleus 

 appears to return completely to its resting stage, and remains 

 unchanged till after the ovum is laid, when again the spindle 

 is formed, heralding in this case the first stage in segmenta- 

 tion. The formation of the polar globules, the fertilization of 



