94 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
chiefly 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 
(1897) 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 ап 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 
be 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. Тһе echinoderms were supposed to pres- 
ent one of the best examples of an indeterminate form of cleavage; fragments of 
the ege 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 .Szrozgylocentrotus, апа presumably in other 
echinoderms also,' 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 
(1895, 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,’ 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 results, which will be published elsewhere, show 
that nothing resembing a normal embryo or larva is ever produced from any fragment of an egg which 
