CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 267 
Like tho cells of the endoderm strand just anterior to them, they prob- 
ably become wandering cells. 
in the posterior portion of the trunk region, where before the closure 
of the blastopore the endoderm strand broadens out into a plate of four 
or more cells, the more laterally placed endoderm cells move dorsad 
at the closure of tho blastopore, and meet in the median plane under- 
neath the chorda. In this way the endoderm of the trunk region is 
converted into a closed vesicle, pear-shaped and broadest in its anterior 
portions ; at its posterior end it is overlaid by tho chorda and flanked on 
each side by the mesenchymo. 
VII. DISCUSSION OF SOME THEORETICAL 
QUESTIONS. 
The facts presented in the foregoine pages have a certain bearing 
on several questions of general interest. Of these I shall make brief 
reference to, — 1. The origin of the germ layers of Chordates; 2. The 
Coolom theory ; and 3. The ancostry of Chordates. 
A. Origin or the Germ Layers of Chordates. 
According to the generally accepted doctrine of Haeckel, al! the higher 
metazoa are ultimately derived from a simple cup-shaped or sac-like 
ancestor composed of two cell layers, an inner and an outer, continuous 
with each other at the margin of the cup or sac. The two coll layers are 
ealled the primary germ layers. "The outer layer is known as the pri- 
mary ectoderm; the inner, as the primary endoderm. Among the Chor- 
dates, this supposed ancestral condition is most nearly realized in ontogeny 
in the case of Amphioxus. The homologues of its inner and outer germ 
layers are traced by embryologists through all the groups of the chor- 
date phylum. A third or middle layer, derived from one or both of the 
others, makes its appearance between the two primary germ layers in all 
the higher Metazoa. Whether this middle layer, or mesoderm, is homol- 
ogous throughout the different groups of Metazoa is one of the most 
difficult and disputed questions in the whole realm of comparative em» 
bryology. Into this question I do not propose to go in this paper; I 
shall confine my attention to the mesoderm of Chordates. 
It is commonly believed that the mesoderm of Chordates is derived 
entirely from tho inner germ layer, which is aecordingly often referred to 
as mes-endoderm, With this view, however, my observations on Ciona 
