ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 87 
fully confirm from my own work. Furthermore, it is not improbable that the same 
thing is true of the amphibians. This is in confirmation of Rabl’s view that the 
peristomal mesoderm is palingenetic, the gastral coenogenetic, and the suggestion 
is raised that in all these cases the gastral mesoderm is derived from the peristomal 
through the manner of overgrowth of the dorsal lip of the blastopore (text figs. 
AXXVI-XXXVIII). 
Although I have made no special study of the subject, and cannot therefore 
speak with assurance, I have seen no evidence in favor of Van Beneden and Julin’s 
view that enterocoels are present in ascidians as in Amphzoxus; in this respect I 
am in accord with the more recent students of ascidian development (Davidoff, 
Castle). 
From these comparisons I think it may be safely concluded that there are 
many fundamental resemblances in the early development of Tunicata, Amphioxus 
and Amphibia, and that in consequence of the early differentiation of the ascidian 
есе and embryo and because of the known cell-lineage of some of its important 
organs the development of these animals throws light os the embryology of other 
chordate classes. 
VII. THE ORGANIZATION OF THE EGG.’ 
It is interesting to observe how recent studies of development have led to the 
recognition of morphogenetie differentiations at earlier and earlier stages in the 
ontogeny ; а dozen years ago the germ layers were the earliest differentiations of 
this sort which were generally recognized. It was in the attempt to determine the 
cellular origin of the germ layers that it became evident that the cleavage cells 
themselves were of morphogenetic value. Some of the differentiations of the 
cleavage cells could be traced back to the very first cleavage or even to the 
unsegmented egg; thus the study of cell-lineage led logically and unavoidably to 
the conclusion that the cleavage cells and even the unsegmented egg must be organ- 
ized with reference to the parts and axes of the future animal. 
For our present purposes the organization of the germ cells has reference only to 
such differentiations as are of direct value in the building of the embryo, in other 
words, such as are morphogenetic, and it may be held to include phenomena of 
polarity, symmetry and localization ; it obviously includes other things also, such as 
regeneration and regulation, which are not, however, objects of investigation in this 
work. 
А. POLARITY. 
Fifty years ago Remak showed that the pigmented hemisphere of the frog's 
egg gave rise to the cells of Von Baer’s “animal germ-layer,’ while the white 
hemisphere gave rise to the “vegetative germ-layer.’ Тһе middle of the ecto- 
dermal hemisphere has ever since been known as the animal pole, the middle of the 
endodermal hemisphere as the vegetative (vegetal) pole. It is a remarkable fact 
* А more complete discussion © this subject, oat that portion of it which relates to ex- ` 
perimental work, is reserved for a subsequent paper, only such matters being treated here as are the 
outgrowth of the ‘observations recorded i i ‘the preceding pages. 
3 
