158 GARDINER. [Vol. XI. 



the ovum, the formation of the first spindle and its disappear- 

 ance under certain conditions, present many problems of 

 interest on which I am now at work, and which will form the 

 subject of a future paper. 



Selenka (3) first called attention to a spindle in the uterine 

 ova of Thysanosooii Diesiiigii, which from the description given 

 resembles very strongly, in size and general appearance, that 

 just described in the ova of P. caudatus. He describes in 

 detail the formation of the spindle seen by him, and states that 

 after the equatorial plate of chromosomes is formed, the polar 

 suns fade in distinctness and draw closer to one another, while 

 the chromatosomes melt together, and the whole nucleus returns 

 to its resting stage. Selenka suggests that this partial karyo- 

 kinesis occurs in order to effect a rearrangement of the yolk 

 particles. Lang (4) states that among the Polyclads, he has 

 observed even in the uterine eggs a similar phenomenon, but 

 cannot accept Selenka's explanation as to the object for which 

 this spindle is formed, for no such rearrangement of the yolk 

 globules occurs at this stage. Wheeler (5) has also observed 

 a similar spindle to occur in the uterine ova of Planocera 

 Inquilina, and also describes it as disappearing before ovi- 

 position. All of these authors state that in these Polyclads, 

 the polar bodies are formed and the fertilization of the ovum 

 effected after ovipositio7t ; therefore the spindle in these forms 

 cannot be the same in origin as that just described in Poly- 

 choerus, in which form the polar bodies are formed before ovi- 

 position. 



The early stages of segmentation may be studied without 

 removing the ova from the capsule, by placing the whole cap- 

 sule in a concave slide. This method is, however, unsatisfac- 

 tory, for it is impossible to rotate the ovum under inspection. 

 When taken from the capsule, which is easily done with 

 needles under a dissecting microscope, the ova generally seg- 

 ment abnormally, and soon die. All of the ordinary killing 

 reagents such as Perenyi, corrosive sublimate, and chromic acid 

 in its various combinations as recommended by different 

 authors, are perfectly useless for these ova, for any of these 

 reagents destroy all trace of segmentation. The most effective 



