236 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



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with reproducing (in Plate IX.) the figures of Plate I. accompanying 

 that paper, which were executed to illustrate this period of the develop- 

 mental history. These figures present dorsal and ventral views of the 

 24-cell stage (Plate IX. Figs. 51 and 52), the 32-cell stage (Figs. 53 

 and 54), and the 46-cell stage (Figs. 55 and 56). 



The interpretation, as given in my preliminary paper, of the lineage 

 through the 46-cell stage rested upon the strongest possible evidence, 

 viz. the observation of karyokinetic figures for every cell division which 

 was represented as having occurred. Moreover, it was shown that these 

 observations made it possible to reconcile the conflicting statements of 

 others who had studied the cleavage of the ascidian egg. Such excellent 

 observers as Van Beneden et Julin, on the one hand, and Seeliger, on the 

 other, held contrary opinions as to which was the dorsal side and which 

 the anterior end of the embryo in its early stages in one and the same 

 genus, Clavelina. 



It was shown in my paper, both from an examination of the authors' 

 own figures and from a comparison with the lineage of Ciona, that their 

 conflicting statements arose from a fundamental error on the part of 

 each, Van Beneden et Julin being correct in their determination of the 

 ends of the embryo, and Seeliger in his determination of the dorsal and 

 ventral surfaces of the early stages. Upon correcting these mistakes, it 

 was found that the observations of the writers mentioned were brought 

 into harmony, and were then in agreement with my own observations 

 on Ciona. 



In order to demonstrate that I had correctly determined the dorsal and 

 ventral faces of the egg for the 46-cell and earlier stages, in contradiction 

 to the interpretation of Van Beneden et Julin, I figured a single older 

 stage described as one of 66 cells (Castle '94, Plate II. Figs. 11 and 12). 

 Its presentation was intended to bridge the gap between the 46-cell stage 

 and gastrulation. This purpose it fulfilled, for it showed gastrulation 

 already commenced, and so proved beyond question which was to be the 

 oral (dorsal) and which the aboral (ventral) surface. 



A desire to give completeness to my figures led me to state the lineage 

 of this stage as I then understood it. I have since found, from the study 

 of more complete series of embryos than I had at that time secured, 

 that I was mistaken as to the time of cell division in one pair of cells 

 (C- s , D 1 - 8 , Fig. 56, Plate IX.). I supposed it had already occurred 

 at the stage represented in Figures 11 and 12 (Plate II.) of my former 

 paper. Consequently the lineage there given for this stage is incorrect. 

 Though this fact does not affect the main conclusions of my preliminary 



