Mosaic Development in Ascidian Eggs. 155 



Cynthia may always be determined renders unnecessary such 

 experiments as Chabry's on the individual cleavage cells. 



If one desires to trace with accuracy the lineage of individual 

 blastomeres, whether in normal or experimentally altered develop- 

 ment, it is essential that a large quantity of material should be 

 available. In even the most favorable material the lineage of the 

 later stages can be successfully studied only by the aid of fixed 

 and stained material and without a large number of eggs it is 

 difficult if not impossible to secure all the stages of development. 

 Furthermore it is desirable that a considerable number of eggs of 

 every stage be available for study, since the liability to error 

 decreases with the number of cases studied. Accordingly, in 

 addition to the study of living eggs during successive stages after 

 their injury, many eggs were also fixed at brief intervals and were 

 afterward stained and mounted entire or sectioned. For this 

 purpose I have found Kleinenberg's picro-sulphuric acid followed 

 by my picro-haematoxylin to give the best results. Entire eggs 

 so prepared show cell outlines, nuclei and karyokinetic figures 

 much more plainly than in the living condition; on the other hand 

 the yellow crescent is less distinct since the yellow pigment is 

 extracted by alcohol; nevertheless this crescent may always be 

 recognized by its peculiar staining qualities and it therefore 

 affords a never tailing aid in orientation. 



III. RESULTS OF EXPERIMENTS. 



In undertaking this work it seemed to me scarcely possible that 

 all of these strikingly different kinds of ooplasm, each with its 

 own peculiar developmental history and destiny, were neverthe- 

 less morphogenetically alike, as might be concluded from the 

 results of Driesch and Crampton. On the other hand a possible 

 escape from this conclusion was suggested by the fact that although 

 the cleavage cells are strikingly different from one another, the 

 isolation of the ooplasmic substances in them is not quite com- 

 plete; almost all of the yellow protoplasm is contained in the yellow 

 crescent; but a small amount of it is found around the nuclei of 

 all the cells; most of the gray substance is contained within the 

 dorsal hemisphere, but a small amount of it occurs in the ventral 

 cells also; most of the clear protoplasm is found in the ventral 

 hemisphere but a small quantity is also found in the dorsal cells. 



