7© GARDINER. [Vol. XV. 



the first cleavage spindle was forming. These experiments 

 were repeated several times with about the same results. 



The sections in no case gave very satisfactory preparations, 

 for frequently the tissues and the ova were abraded and injured 

 by the severity of the experiment. It was, however, clearly 

 demonstrated that if the animal bearing ripe ova were placed 

 under such conditions that it either could not, or would not, lay 

 its eggs, the development of the ova continued much as if 

 normal oviposition had taken place. 



In several ova a larger spindle strongly resembling the disap- 

 pearing spindle as shown by Lang (Tafel 20, Fig. 4) was found. 

 I would suggest, therefore, that individuals of the Polyclads, in 

 which such structures are found, have before death been placed 

 under some abnormal conditions ; that the ovum has been ferti- 

 lized and the polar bodies formed ; that the first segmentation 

 spindle has been formed ; and that the environment was such 

 that oviposition could not take place ; consequently, that a retro- 

 grade development of this spindle has taken place exactly as 

 in Polyclioeriis. If this is so in one Polyclad, it may certainly 

 be so in others, and it seems much more logical to assume that 

 under fitting conditions the first segmentation amphiaster may be 

 formed in the uterine ovum and then undergo retrograde devel- 

 opment, as I have demonstrated to occur in P. caudatus, than 

 that an amphiaster should be formed with the express purpose 

 of disappearing again, as the observation above quoted would 

 indicate. The figures in von Graff's (9) great work show 

 in animals of several species ripe ova with large amphiasters 

 already formed within them. Von Graff, however, does not 

 discuss this matter in the text. It is interesting to note 

 the parallelism in the extrusion or non-extrusion of the polar 

 bodies in these two forms. In both, the polar bodies are 

 extruded only when the ovum has been laid before these 

 bodies are formed. If, for any reason, the polar bodies are 

 formed before the eggs are laid they are not extruded. It 

 may be that the sea water comes in more intimate contact 

 with the egg in the capsule than when within the parent, and 

 in some way stimulates the ovum to extrude the bodies. 



