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 rases the gastral mesoderm is derived from the peristomal 

 through the manner of overgrowth of the dorsal lip of the blastopore (text figs. 

 XXXY1 XXXVIII). 



Although 1 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 thai enterocoels are present in ascidians as in Amphioxus\ in this respect 1 

 am in accord with the more recent students of ascidian development (Davidoff, 

 Castle). 



From these comparisons 1 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 

 egg and embryo and because of the known cell-lineage of some of its important 

 organs the development of these animals throws light upon the embryology of other 

 chord ate classes. 



VII. THE ORGANIZATION OF THE EGG. 1 



It is interesting to observe how recent studies of development have led to the 

 recognition of morphogenetic differentiations at earlier and earlier stages in the 

 ontogeny; a 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 havers 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 ver} r 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 lias 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. 



A. 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." The middle of the ecto- 

 dermal hemisphere hasever since been known as the animal pole, the middle of the 

 endodermal hemisphere as the vegetative (vegetal) pole. It is a remarkable fact 



1 A more complete discussion of this suhjert, especially that portion of it which relates to ex- 

 perimental work, is reserved for a subsequent paper, only such matters being treated hen- as are the 

 outgrowth of the observations recorded in the preceding pages. 



