94 ORGANIZATION AND (ELL-LINEAGE OF ASCIDIAN EGG. 



chief!} employed in experimental work, the cleavage was not known to be constant 

 and differential in character; whereas in all forms the cell-lineage of which was 

 known, the cleavage was both constant and differential. I therefore suggested 

 i ! "- '. ' 7 1 that for the present it would be advisable to recognize two types of cleavage, 

 a determinate type in which the blastomeres are differentiated from one another 

 and are constant in their manner of origin and development, and an indeterminate 

 type.in which such differentiation and constancy are not known to occur. At the 

 same time I was careful to state that this indeterminateness might be only apparent 

 and not real, and "that the denial of a definite prospective value to each blasto- 

 mere might rest upon the curious basis that no one had followed a single blasto- 

 mere through the development" (1897, p. 191). In favor of such a distinction was 

 the experimental work which had been done on the eggs of ctenophores and gaster- 

 opods ; the cleavage in these animals is known to be determinate, and it was found 

 that from a part of an egg only a part of an embryo would develop. In all cases 

 constant and differential features appear sooner or later in the course of develop- 

 ment, but if in some cases they appear late in the cleavage while in others they 

 appear early this would explain the fact that in some species a whole embryo may 

 he produced from one of the first two or first four blastomeres, whereas in other 

 cases only a partial embryo results. Wilson in particular has defended the view 

 that specifications arise at different times in different eggs, and that these differ- 

 ences in the time of specification may explain the different potencies of blastomeres 

 or portions of the egg. 



While it is entirely possible that differentiations may appear in some cases 

 earlier than in others, experiments on the development of parts of eggs are no 

 satisfactory test of the presence or absence of such differentiations as the eggs of 

 echinoderms and ascidians well show. The echinoderms were supposed to pres- 

 ent one of the best examples of an indeterminate form of cleavage; fragments of 

 the egg or isolated blastomeres here give rise to entire embryos, and it was conclud- 

 ed that differentiations must appear in these eggs relatively late in development. 

 But Boveri (1901) has shown that in Strongyloceiitrotus, and presumably in other 

 echinoderms also, 1 a remarkable stratification of the egg. corresponding to the pri- 

 mary organs of the larva, appears at the time of the maturation of the egg. These 

 observations have taught us more with regard to the actual differentiations of this 

 egg. as contrasted with the potencies of its parts, than all the experiments which 

 have ever been made. Again, the ascidian egg has one of the most determinate 

 and morphogenetic forms of cleavage known and the differentiations of the various 

 parts of the unsegmented egg are very great, and yet the experiments of Driesch 

 i L895, 1903) and Crampton (1897) have shown that entire embryos may be pro- 

 duced from isolated blastomeres of this egg ; such experiments apparently demon- 

 strate the totipotence of the first four blastomeres of the ascidian egg, 2 but all the 



See foot-note, p. 89. 



! Since this paper was written I have carefully studied the potency of individual blastomeres of 

 the ascidian egg by the experimental method. My result-, which will be published elsewhere, show 

 that nothing resembing a normal embryo or larva is ever produced from any fragment of an egg which 



