Mosaic Development in Ascidian Eggs. 151 



larva within the egg membranes, and about twelve hours after the 

 fertilization of the egg the larva may hatch and become tree 

 swimming. However, in a considerable proportion of cases the 

 larva never hatches but undergoes its metamorphosis within the 

 egg membranes. 



II. OBJECTS AND METHODS OF EXPERIMENT. 



This brief review of the normal development^ shows that there 

 is a remarkable degree of differentiation and localization ot the 

 substances of the egg and embryo and it seems to render necessary 

 some further explanation of the results of the experiments of 

 Driesch and Crampton; certain it is that the egg is highly differ- 

 entiated and if portions of this differentiated ooplasm may give 

 rise to portions of the larva which they would never produce under 

 normal conditions it is important to know the steps by which this 

 is accomplished. 



With this object in view I spent the summer of 1904 at the Ma- 

 rine Biological Laboratory at Woods Hole, Mass., experimenting 

 on the eggs of Cynthia (Styela) partita and of Molgula man- 

 hattensis; I was unable to obtain Ciona intestinalis, the normal 

 development of which I had studied during the previous summer, 

 and my experimental w^ork is therefore limited to the two species 

 first named. Most of my work was done on the egg of Cynthia, 

 which is a better object for experimental work than that of Mol- 

 gula, owing to its greater size and the more brilliant coloring of its 

 different ooplasmic substances. Enough work was done on Mol- 

 gula, however, to show that the development of isolated blasto- 

 meres is the same in this genus as in Cynthia. 



All the experiments performed had for their purpose the testing 

 of the potencies of the various substances and blastomeres of the 

 egg. Injuries to the unsegmented egg of whatever nature, 

 whether produced by sticking, cutting or shaking the eggs, 

 invariably inhibited all further development. I have therefore 

 been unable to test the developmental potencies of the different 

 kinds of ooplasm of the unsegmented egg. But inasmuch as these 

 substances are the same in appearance and localization before and 



'For a more detailed account of the normal development of these ascidians the reader is referred 

 to my previous papers on the "Organization and Cell-Lineage of the Ascidian Egg" ("05'), and 

 on "Organ-Forming Substances in the Eggs of Ascidians" ('05-). 



