Mosaic Development in Ascidian Eggs. 215 



the planes of cleavage bear no constant relation to the planes of 

 localization, as in the eggs of some of the vertebrates (frog, fish). 

 For all such cleavages I proposed the name "indeterminate," but 

 at the same time I was careful to state that this was "to be under- 

 stood as applying only to the cleavage, for in its main features and 

 results the development of all animals is determinate; that is, 

 predictable. Even in cnidaria, echinoderms and vertebrates 

 there appears successively a blastula, gastrula, larva, and adult 

 of determinate form and character" ('98, p. 21). 



But w^hile the cleavage is indeterminate in some cases there is 

 reason to suppose that there is a definite organization of the egg 

 in all animals — in short that the organization of the individual is 

 determinate at all stages from the egg to the adult. Even in such 

 an egg as that of Pennaria it is certain that there must be deter- 

 minative factors somewhere, if not in the cytoplasm then in the 

 nucleus, which determine that the egg shall develop into a Pennaria 

 rather than into some other animal; and it is further evident that 

 these determinative factors must be present in the cytoplasm at a 

 relatively early stage, if not at the very beginning of development. 



In the echinoderm egg, which was at one time supposed to be 

 homogeneous or isotropic, Boveri ('01) has shown that a polar- 

 radial localization of at least three distinct morphogenetic sub- 

 stances takes place immediately after maturation, and in this case, 

 as in the ascidians it is probable that there is an earlier concentric 

 localization of these substances in the oocyte. Since these three 

 substances are localized in zones or strata, one above the other, 

 around the chief axis of the egg, they are all present in each of the 

 first four blastomeres of the egg, each of which may give rise to an 

 entire embryo; but when they are isolated each is found to be 

 strictly limited in its potentialities. 



While therefore there are several groups of animals in which 

 the cleavage is indeterminate there are few or none in which the 

 ooplasm is isotropic; on the contrary in almost every phylum the 

 eggs and blastomeres show differentiations and localizations of the 

 ooplasm which are of morphogenetic value. "Everywhere," as 

 Fischel ('03) has well said, "the fundamental principle of normal 

 development is a mosaic work." But while Fischel supposes that 

 "only the materials for the primitive organs of the embryo are 

 preformed in the egg cell and that the material substratum for the 

 differentiation of the special organs is probably first formed during 



