CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 261 
'To summarize our observations on the series of sections just examined 
(Figs. 93-97): — 
(1) The fundament of the nervous system consists of a medullary plate 
extending from near the anterior end. of the embryo to the blastopore, 
and continued backward by cells lying on each side of the blastopore 
and along the line where the lips of the blastopore have fused. The 
transformation of the medullary plate into a medullary groove proceeds 
from the blastopore forward. 
(2) The chorda fundament consists of a plate of cells immediately 
underneath the medullary plate, but extending neither so far forward 
nor so far backward in the embryo. A part of it lies on each side of the 
biastopore, but the larger part is anterior to the blastopore, 
(3) The mesenchyme extends in two lateral bands from the region of 
the blastopore forward through about two thirds of the extent of the 
embryo anterior to the blastopore. 
(4) The muscle cells lie principally posterior to the blastopore in a 
pretty compact mass. They extend no farther forward than the first 
section anterior to the blastopore. 
(5) The endoderm consists of a double row of large cells ventrally 
situated extending from the first section behind the blastopore through. 
tho next five anterior sections ; it then broadens out and occupies nearly 
the whole inner layer of the embryo, both dorsally and ventrally, anterior 
to the chorda fundament. 
CG. SUMMARY ON GASTRULATION. 
1. In the gastrulation of Ciona two processes can be distinguished : 
(a) a progressive invagination of the cells on the dorsal surface of tho 
embryo, beginning at its centre ; (P) a concomitant, overgrowth of cells 
from the ventral side of the embryo, caused by more rapid cell division 
in that region. The overgrowth is greater at the anterior than at the 
posterior end of the embryo, because cell division proceeds more rapidly 
at the anterior end. 
2. Karly in the process of gastrulation one can recognize a ring of 
cells encircling the blastopore peculiar in their stainability, forming tho 
common fundament of the nervous system and the longitudinal muscu- 
1 
lature of the larva’ Anterior to the blastopore the ring broadens out 
1 The existence of this peculiar ring of cells was first pointed out by Van Bene- 
den et Julin ('80) in the case of Clavelina ; but these authors made the mistake of 
regarding it as exclusively nervous. 
