206 PROCEEDINGS OF THE AMERICAN ACADEMY. 



These figures leave no doubt that the view of the Ciona egg given 

 in P^igure 3 corresponds with that of" the egg of Claveliua given in 

 Figure 7, and that the view of the Ciona egg given in Figure 4 cor- 

 responds with the view of the Clavelina egg given in Figure 8. If then 

 it can be sliovvn by a detailed study of later stages that the orientation 

 given for the egg of Ciona in Figures 3 and 4 is the correct one, it 

 follows that the orientation given for the egg of Clavelina in Figures 

 7 and 8 is also correct, and that the orientation given to this stage by 

 Van Beneden et Julin is consequently wrong; for they call Figure 7 

 a dorsal, and Figure 8 a ventral view. 



An examination of Figure 3 shows that the thin superficially large 

 cells seen in this view of the egg are preparing for division. These, 

 as the lettering indicates, are derived from the four veutrally situated 

 cells of the 8-cell stage, and will be referred to collectively as the 

 ventral or ectodermal half of the %gg. The latter term must not be 

 understood, however, to imply that all the cells derived from this 

 hemisphere become ectoderm, but merely that ectoderm is its prin- 

 cipal derivative. 



The other half of the egg, composed of high, columnar cells, will be 

 called the dorsal or endodermal half of the egg, with the same restric- 

 tion on the term endodermal as has been made for ectodermal. 



Figure 4 shows the cells of the endodermal half to be quiescent, 

 while those of the ectodermal half are preparing for division. A 

 similar acceleration in division of the ectodermal over the endodermal 

 half of the egg was to be observed at the 24-cell stage (Figs. 1 

 and 2). Here the cells of the ectodermal half are seen to have passed 

 into the sixth generation, while the cells of the endodermal half are 

 only in the spindle stage, preparatory to the division which will carry 

 them into that generation. 



Figure 5 represents a ventral view of a stage of forty-six cells, and 

 shows accomplished the divisions foreshadowed in Figure 3. All 

 the cells of the ventral half of the egg, except the pair of small 

 posterior cells, have divided either completely or partially, thus pass- 

 ing into the seventh generation. I should add that no one of the 

 divisions indicated is a matter of inference, but spindles have been 

 observed in every instance ; where they were not to be seen in the 

 specimen figured, they have been observed in other specimens not 

 quite so advanced in development. 



Figure 6 gives a view of the opposite face of the same egg. The 

 cells of the dorsal half are here seen to be in the sixth generation, 

 and still quiescent. In a single cell, b®•^ a pair of astral radiations 



