60 



can be detected in the mature ova, thougli it is readily seen 

 in tlie immature. 



The Ascidians are true hermaphrodites, and artificial im- 

 pregnation can readily be conducted with ova and sperma- 

 tozoa taken from the same animal. The spermatozoa attach 

 themselves to the ova not by their heads but by their tails. 

 Their entrance into the ovum was not observed. 



Cleavage commences within an hour of impregnation, and 

 advances very rapidly. It is confined entirely to the vitellus, 

 the gelatinous layer Avith its vesicles taking no share in the 

 process. Two meridian furrows divide the ovum first into 



two then into four masses, which are further divided into 

 eight by an equatorial furrow. Even in the two first masses 

 a nucleus may be detected, and the cleavage masses arrange 

 themselves around a central cleavage cavity (cavity of Baer). 



As soon as the number of cleavage masses amounts to 

 about thirty-two the ovum begins to be flattened below, and 

 then is doubled in on itself. Through this involution the 

 ovum becomes gradually transformed into a double sac, the 

 cells from the upper set of cleavage masses, numerous and 

 small, forming the outer sac, while those from the lower set, 

 fewer and larger, forms the inner or lining sac, the original 

 cleavage cavity being reduced to a narrow space betAveen 

 the two, as is shoAvn in fig. 1 c. The new cavity thus 

 formed corresponds to the alimentary cavity of the future 

 animal {h) ; at first it is of the form of a shallow cup, but it 

 gradually becomes deeper and its orifice narrower, until at 

 last if the external opening does not close up all together it 

 becomes so small as to escape notice. 



The cells constituting the inner sac or walls of the ali- 

 mentary cavity now increase more rapidly, and the ovum at 



