858 Aspects of the Body in Vertebrates and Arthropods, gao cali 
250-252) or that of other observers. While the nervous 
of all animals arises from the ectoderm (epiblast), as Balfour : 
Chordata 
states: “In all 
| as the medullary plate, — 
1-Early stage of Ascidian embryo, show- becomes isolated from the 
ing the nervous tube #, open in front and sinata remainder of the layer to 
dors vow 4 above the a sea pat tube (4), asi the central 
tebrates give rise to 
nervous axis ;” in tunicates as well as vertebrates this plate is con- 
verted into a tube or canal, which lies wholly above the alimen- 
tary tract. It is this striking feature in embryo tunicates which 
mainly seems to justify their elimination con the worms and 
cture ; the 
Pia: 2.—Embryo of an Pree showing the vertebrate plan of on sore 
nervous system 4’, 4 with the spinal nerves (s) being situated dorsally 
notocord (c) and A odotaa anal (ġ, 7). 
ams pea 
indicates their proximity to the vertebrates, as this "o 
more truly vertebrate feature than even the possession 0! ofa e 
cord. ~ = 
Balfour states on p. 342° a 
spinal cord, gps after the 
sure of the medullary 
in all the true Vertebrata, pe 
form of an oval tube; the W 
of which are of a -= «d d 
Ci 
= ate of ven 
an axial strip of the dorsal — 
epiblast, extending from 
the lip of the blastopore — 
to the anterior extremity 
of the head, and known — 
zai z ` Š ` á 
EE E Mea a ep T E eee Er E E, a NAN URA 
canal, has, 
E ES EEE N 
