394 ZOOLOGY. 
from the upper part of the germ. This is the central ner- 
vous system, and in the cavity are subsequently developed 
the sense organs. We thus see, says Kowalevsky, a com- 
plete analogy in the mode of origin of the nervous system of 
the Ascidians to that of the vertebrates, the nervous cavity, 
where the embryo is seen in section, being situated above 
the digestive cavity in both types of animals. 
The next important stage is the formation of the tail. 
The pear-shaped germ elongates and contracts posteriorly 
until of the form indicated at Fig. 386’, B. At this period 
appears the axial string of nucleated cells, called the chorda 
dorsalis, as it is homologous with that organ in Amphioxus 
and the embryo of higher vertebrates. The nervous system 
consists of a mass of cells extending halfway into the tail. 
and directly overlying the chorda, but extending far beyond 
the end of the latter as seen in the figure. The nerve-cav- 
ity (B, 1) atter closing up forms the nerve-vesicle, a large 
cavity (Fig. 386°, a), m which the supposed auditory organ 
(e) and the supposed eye (a) arise ; this cavity finally closes, 
and the sense-organs are indicated by certain small masses 
of pigment cells im the fully grown Ascidian larva. 
As the embryo matures, the first change observed in the- 
cord is the appearance of small, refractive bodies between 
the cells. Between the neighboring cells soon appear in the 
middle minute highly refractive corpuscles which increase 
in size, and press the cell-contents out of the middle of the 
cord. After each reproductive corpuscle grows so that the. 
central substance of the cell is forced out, it unites with 
the others, and then arises in the middle of the simple cel- 
lular cord a string of bodies of a firm gelatinous substance 
which forms the support of the tail. After this coalescence 
the substance develops farther and presses out the proto-: 
plasm of the cells entirely to the periphery. The cord when 
complete consists of a firm gelatinous substance surrounded 
by a cellular sheath which is formed of the remains of the 
cells originally comprising the rudimentary cord. The cells 
lying under the epithelial layer form a muscular sheath of 
which the cord (Fig. 386°, ¢) is the support or skeleton. 
The alimentary cavity arises from the primitive cavity 
