172 EDOUARD VAN BExVEDEN. 



time to time the embryonic development advances rapidly, 

 and at the end of from two to three days the ciliated em- 

 bryos swim freely in the water. Artificial fecundation was 

 performed for the first time in an Echinoderm by K. E. Von 

 Baer ; after him several embryologists have had recourse to 

 the same procedure in order to study the development of 

 Echinida, Asterida, and Holothurida. I will mention only 

 Derbes, Krohn, Busch, J. Miiller, A. Agassiz, and 

 Selenka. 



If, some few seconds after having performed artificial 

 fecundation, one places a certain number of ova on an object 

 glass, having taken them with a pipette from the bottom of 

 the vessel in which the sexual products have been mixed, 

 one observes that a crowd of spermatozoa have collected to- 

 gether at the surface of the zona pellucida. They move their 

 tails with such force, that they even succeed in making the 

 ova move. If, in order to study the successive phenomena 

 which take place in it, one chooses an ovum which presents 

 a superficially situated germinal vesicle, and if one watches 

 it continuously, one finds that three quarters of an hour or 

 an hour after the fecundation, the germinal vesicle which avhs 

 so distinct at the time of the commencement of the obser- 

 vation, has completely disappeared. Subsequently one sees 

 the vitellus undergo in succession the phenomena of retrac- 

 tion ; distinctive bodies and polar globules appear in the 

 peri-vitelline fluid ; then the primary vitelline globe breaks 

 into two parts. The ova, therefore, which are provided 

 with a superficial germinal vesicle, are fit and liable, as well 

 as those which no longer showed any traces of it in the 

 ovary, to be fecundated. In order to be sure whether the 

 disappearance of the germinal vesicle is the consequence of 

 fecundation, or whether it occurs independently of the action 

 of the semen, it is sufficient to turn some ovarian eggs into 

 another vessel, taking care to avoid all mixture with the 

 spermatic fluid. If one observes under the conditions above 

 described ova taken from the bottom of the vessel, one can 

 follow and observe the successive phases of the disappear- 

 ance of the vesicle, just as when one follows them in fecun- 

 dated ova. This disappearance then is independent of the 

 action of the spermatozoa. This conclusion might, moreover, 

 be drawn from this iact, that some of the ova lose their 

 germinal vesicle, whilst still in the ovary, and that neverthe- 

 less these ova are perfectly fertile. 



It was of the highest importance to study the successive 

 phases of the disappearance of the vesicle, in order that 

 we might be able to determine with certainty how it 



