146 Edwin G. Conklin. 



ual blastomeres throughout the development. The most notable 

 exception to this rule is found in the case of ascidians. That the 

 cleavage of the egg in these animals is constant in form and differ- 

 ential in character and that specific blastomeres are destined in 

 the course of normal development to give rise to specific parts of 

 the larva has been demonstrated by Van Beneden and julin, 

 Chabry, Castle, and many others. Chabry ('87) also showed, 

 in one of the earliest experimental investigations dealing with the 

 potency of cleavage cells, that individual blastomeres of Ascidia 

 aspersa always develop into those parts of the larva which they 

 would produce under normal conditions. On the other hand, 

 Driesch ('95) discovered, some eight years later, that in Phallusia 

 mammilata individual blastomeres up to the 4-cell stage at least 

 are capable of giving rise to entire larvae and this conclusion was 

 afterward confirmed by Crampton ('97) in the case of Molgula 

 manhattensis. Since the results of Chabry were thus flatly con- 

 tradicted by these later investigators and as they have been de- 

 fended by no one who has actually experimented on these eggs^ 

 these results have been generally discredited and the ascidians 

 are now commonly regarded as belonging to that group of animals 

 in which the early cleavage cells are equipotential. The ascidians, 

 therefore, should afford an excellent opportunity of determining 

 the exact method by which an egg fragment or isolated blasto- 

 mere gives rise to an entire larva, since in this case it is possible 

 to follow the lineage of individual cells until they enter into larval 

 organs; furthermore, they should afford means of testing the 

 justice of the distinction which has been proposed (Conklin, '97) 

 between determinate and -indeterminate types of cleavage, and 

 finally they should throw light upon the significance of the high 

 degree of differentiation which is known to exist in the early 

 development of these animals. 



I. NORMAL DEVELOPMENT. 



I have recently ('05') shown that these differentiations of the 

 ascidian egg are much greater than has heretofore been supposed; 

 in the unsegmented egg of Cynthia (Styela) partita at least five 

 distinct kinds of ooplasm can be recognized. These are, (i) the 



'Several persons, viz: O. Hertwig ('92), Rous ('92), Weismann ('92), Barfurth C93) have discussed 

 Chabry's work from a critical point of view. 



