A THREE-EYED REPTILE. 
259 
following the life-history of certain species of Ascidia from 
the egg onwards, and the course of development is such as to 
leave no doubt but that these animals possess a median 
dorsal structure homologous with the “back-bone” of the 
Vertebrata. The Ascidia and Vertebrata ought, therefore, to 
be placed together in one class, the former being removed 
from their position near the Mollusca, with which animals 
they have no affinity. As, however, the back-bone of an 
adult fish or mammal is divided into a number of segments 
or vertebra , whilst that of the Ascidia is not, the latter 
could not be classed as “ Vertebrata,” therefore all animals 
possessing a back-bone at all have been classed as “ Chordata,” 
from xopS? 7 , a string of a lyre. The term “Vertebrata” is 
restricted to those animals which possess a segmented back¬ 
bone in adult life, though it is an important fact that in all 
of them when embryos it is gelatinous and unsegmented as 
it is in the Ascidians. Hence, in this condition, in both 
groups of animals, it is called the notochord, from v&tov, 
the back. On the other hand, as the notochord is present 
in the tail only of the Ascidia, they have been termed 
“ Urocliorda,” from 6vp&, the tail. It may be mentioned 
in passing that the curious degenerate Lancelet is also 
placed in a separate group of the Chordata. 
The egg of the Ascidian is a simple cell, and after fertili¬ 
sation becomes segmented; that is to say, it becomes divided 
first into two halves, then each of these becomes similarly 
divided, and so on, until at last when the process is complete 
we have a little mass or morula composed of a great many 
cells, from which the various organs of the Ascidian will be 
built up. After various changes our embryo becomes pear- 
shaped and hollow, having an aperture at the posterior end, 
the cavity being the primitive alimentary canal. Its dorsal 
surface now becomes flattened and grooved in a line parallel 
to the long axis of the embryo, the medullary groove being 
thus formed. The edges of the groove thicken and soon 
grow up over it, finally coalescing with each other along its 
whole length, thus forming a tube of cells—the spinal cord — 
enclosing the medullary canal. Meanwhile the notochord has 
been formed near the posterior end of the embryo, between 
the spinal cord and the primitive alimentary canal—that is, 
in exactly the same position as the back-bone in a man. 
It does not, however, extend into the trunk of the animal, but 
is confined to the tail which is now formed, and which 
continues for some time to elongate. 
The development of the nervous system proceeds by the 
enlargement at its anterior end of the medullary canal 
