970 REPORT—1896. 
region an afterthought, that the attempt so often made to find vertebre and spinal 
nerves in the cranial region is an attempt to put the cart in front of the horse—to 
obtain youth from old age. Wemay, it seems to me, fairly argue from the sequence 
of events in the embryology of: vertebrates that the primitive vertebrate form was 
chiefly composed of the head region, and that between the head and the tail was a 
short body region. In other words, the respiratory chamber and the cloacal region 
were originally close together, just as would be the casein Limulus if the branchial 
appendages formed a closed chamber. According, then, to my view, there would 
be no difficulty in the respiratory chamber opening originally into the cloacal 
region, z.c. the same cloacal yegion into which the neurenteric canal already 
opened. The short junction tube thus formed would naturally elongate with the 
elongation of the body, and, as it originally was part of the respiratory chamber, 
it equally naturally is innervated by the vagus nerve. This, then, is the explana- 
tion of that most extraordinary fact, viz. that a nerve essentially branchial should 
innervate the whole of the intestine except the cloacal region. Whether this is the 
true explanation of the formation of the mid-gut of the vertebrate cannot be tested 
directly, but certain corollaries ought to follow: we ought to find, on the ground 
that the sequence of the phylogenetic history is repeated in the embryo, that, 
1, the growth in length of the embryo takes place between the cranial and sacral 
regions by the addition of new segments from the cranial end; 2, the formation 
of the fore-gut and hind-eut ought to be completed while the mid-gut is still an 
undifferentiated mass of yolk cells; 3. the cloacal region ought to be innervated 
from the sacral nerves, while the stomach, mid-gut and its appendages, liver and 
pancreas, ought to be innervated from the vagus. 
The first proposition is a well-known embryological fact. The second pro- 
position is also well known for all vertebrates, and is especially well exemplified in 
the embryological development of Ammoccetes, according to Shipley. The third 
proposition is also well known, and has received valuable enlargement in the recent 
researches of Langley and Anderson.! Further, we see that in this part of the 
body the ancestor of the vertebrate must have had a ccelomic cavity the walls of 
which were innervated, not from the mesosomatic nerves or respiratory nerves, but 
from the metasomatic group of nerves; and in connection with this body cavity 
there must have existed a kidney apparatus, also innervated by the metasomatic 
nerves; with the repetition of segments by which the elongation of the animal was 
brought about the body cavity was elongated, and the kidney increased by the 
repetition of similar excretory organs. All, then, that is required in the original 
ancestor in order to obtain the permanent body cavity and urinary organs charac- 
teristic of the vertebrate is to postulate the presence of a permanent body cavity in 
connection with a single pair of urinary tubes in the metasomatic region of the 
body. As yet I have not worked out this part of my theory, and am therefore 
strongly disinclined to make any assertions on the subject. I should like, however, 
to point out that, according to Kishinouye,”? a permanent body cavity does exist in 
this part of the body in spiders, known by the name of the stercoral pocket; into 
this coelomic cavity the excretory Malphigian tubes open. 
The Paleontological Evidence, 
It is clear, from what has already been said, that the paleeontological evidence 
ought to show, first, that the vertebrates appeared when the waters of the ocean 
were peopled with the forefathers of the Crustacea and Arachnida, and, secondly, 
the earliest fish-like forms ought to be characterised by the presence of a large 
cephalic part to which is attached an insignificant body and tail. 
Such was manifestly the case, for the earliest fish-like forms appear in 
the midst of and succeed to the great era of strange proto-crustacean animals, 
when the sea swarmed with Trilobites, Eurypterus, Slimonia, Limulus, 
Pterygotus, Ceratiocaris, and a number of other semi-crustacean, semi-arachnid 
1 Langley and Anderson, Journ. of Physiology, vols. xviii. xix. 
2 Kishinouye, Journ. of Coll. of Sci, Tokio, vol. iv. 1890, vol. vi. 1894, 
