No. I.] POLYCHOERUS CAUDATUS. 77 



While seeking to compare the above described retrograde 

 growth of the amphiaster in the ova of an Acoela with the 

 " disappearing spindle " in the uterine ova of a Polyclad, the fol- 

 lowing facts should be borne in mind. In both of these groups 

 the spermatozoa to be used in fertilizing the ova are contained 

 within the egg-bearing animal, introduced, probably, in most if 

 not in all cases, by hypodermic impregnation. (See Lang, 

 p. 636.) In the Polyclads the normal process is that, when the 

 eggs are laid, one or two spermatozoa pass with each ovum into 

 the egg capsule, where fertilization takes place later ; hence, if 

 for any reason oviposition is delayed or prevented, it would 

 seem perfectly possible that fertilization might be effected in 

 tttero. That such is the case under some circumstances the 

 following experiments show conclusively. 



In the summer of 1895 and again in 1896 I obtained quite 

 a number of Leptopla7ia variabilis (Verrill), which laid quite 

 freely in captivity. In each egg capsule examined, from 

 one to three spermatozoa were found. In less than three 

 hours after oviposition the polar bodies were extruded, and 

 in from one to three hours after this the line of the first 

 cleavage plane was evident, and the two-celled stage was soon 

 after reached. 



These facts being established, I brought six specimens which 

 appeared to contain ripe ova into a dish of warm, somewhat 

 stale sea water, in which were a number of Polyclioerus catidatus 

 on which I was experimenting. Within an hour two laid ova 

 in which the development proceeded as above described. At 

 the end of eight hours the ova in the other four had, as far as 

 I could observe, undergone no change. The next morning 

 (twenty hours) the worms were almost dead ; nor did they 

 appear to revive on being placed in fresh sea water. They were 

 then killed and sectioned. In two ova amphiasters were fully 

 formed, which from their small size appeared to be destined for 

 polar body formation rather than for the first segmentation 

 cleavage. Into one of these ova the sperm had penetrated. 

 In another ovum a polar body had been divided off, but not 

 extruded, and a sperm had entered the ovum ; while in another, 

 two polar bodies were lying close to the &gg membrane, and 



