CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 245 
Dorsal hemisphere : — 
2 mesoderm cells in the 7th generation. 
10 chorda i Tth eE 
10 endoderm M 6th N 
22 — 
90 (in ventral hemisphere). 
112 
The process of gastrulation has at this stage already set in. Not only 
is the eotoderm growing over so as to envelop the dorsal hemisphere, 
but the latter is at the same time sinking down and becoming saucer- 
shaped. (Cf. Figs. 66 and 77.) Accordingly, gastrulation may be said 
to take place by a combination of the two processes of epiboly and 
invagination. 
(b) Differentiation of the Principal Organs as seen at the 112-cell 
Stage. 
a. TOPOGRAPHICAL. 
We will now consider this same embryo (Plate XI. Fig. 71) with 
reference to the ultimate fate of its cells. At the depressed centre of 
its dorsal surface, surrounding the point of formation of the polar glob- 
ules, we find the ten cells of the definitive endoderm, all in the sixth 
generation and containing each a very large nucleus. They are a, a**, 
d°5, 498, q*7, and the corresponding cells in quadrants Band C. Two 
of then are derived from each of the anterior quadrants (A and B), and 
three from each of the posterior quadrants (O and D). Together they 
constitute the entire fundament of the definitive larval endoderm. 
The endoderm fundament is surrounded by two concentric rows of 
ells from which are derived some of the most important organs of the 
arva, The inner row or ring of cells we will call the chorda-mesenchyme 
ring, because it is destined to produce the chorda and mesenchyme. In 
it we must include the small flattened cells, O75, D"5, but not their sis- 
ter cells, 07%, D'S, which, though in contact superficially with endoderm 
cells, really belong, as their fate shows, in the second or outer ring. 
The chorda mother cells, all of which are included in the :chorda- 
mesencbyme ring, are derived, as has been already stated, in part from 
the anterior and in part from the posterior quadrants. Those derived 
from the anterior quadrants are at this stage eight in number, They 
form the anterior segment of the chorda-mesenchyme ring (Fig. 71, 
at), at, a8, a and the corresponding cells on the left of the 
median plane). The posterior chorda cells are only two in number 
