314 A. KROHN ON THE DEVELOPMENT OP THE ASOIDIANS. 



duct open, close by one another. Since, however, in the Phal- 

 lusia, we never meet in this cavity with ova whose development 

 has commenced, as is commonly the case in the compound 

 Ascidians, perhaps nothing is more probable than that the ova 

 are cast forth soon after fecundation, and that their development 

 begins first, external to the parent. 



The cleavage of the yelk commences, about two or three 

 hours after the semen has been brought into contact with the 

 ova. It proceeds for the first stages, at least, in a very regular 

 progression. I believe I have made out with tolerable certainty 

 that every cleavage mass, after the completion of its division, is 

 surrounded by an excessively delicate membrane. On the ad- 

 dition of water impregnated with acetic acid, we see this invest- 

 ment gradually raised up from the vitelline contents, which con- 

 tract into a smaller space, as an independent membrane. As 

 regards, however, the clear vesicular nuclei within the cleavage 

 masses, I believe I have arrived at the result, that they dis- 

 appear when a new division impends, and only after this has 

 come to an end do they reappear again, newly formed. In their 

 place there may be observed, in any yelk-mass about to divide, 

 a very peculiar arrangement of the viteUine molecules. The 

 latter are in fact disposed in thick striae, which radiating from 

 the centre, are directed on all sides towards the lighter periphery 

 of the mass, and seem to spread from two centres of irradiation. 

 When, after the completion of division, the nuclei appear within 

 the new cleavage masses, the radiate striation has also dis- 

 appeared, and the yelk-granules are seen, lying close to one 

 another in the masses, without any definite arrangement. All 

 these appearances agree, however, with that later view of the 

 cleavage process, which was first taken by Riechert (Mull. 

 Archiv, 1846, p. 196). 



The glassy layer upon the yelk, which we have just described 

 as the original rudiment of the test present in the unfecundated 

 ovum, takes not the smallest share in these thorough changes 

 of the yelk; it remains, without undergoing any alteration, 

 closely apposed to the vitelline membrane. 



Before the expiration of the first four-and-twenty hours after 

 fecundation, we find in most ova that the embryo has assumed 

 its well-known cercarian form, with a more or less developed 



