The Evolution of Man, vol 2 | Page 8

differing in contents. In the female they
contain a quantity of simple ova (Figure 2.219 g); in the male a number
of much smaller cells that change into mobile ciliated cells
(sperm-cells). Both sacs lie on the inner wall of the atrium, and have no
special outlets. When the ova of the female and the sperm of the male
are ripe, they fall into the atrium, pass through the gill-clefts into the
fore-gut, and are ejected through the mouth.
(FIGURE 2.216. Transverse section of the lancelet, in the fore half.
(From Ralph.) The outer covering is the simple cell-layer of the
epidermis (E). Under this is the thin corium, the subcutaneous tissue of
which is thickened; it sends connective-tissue partitions between the
muscles (M1) and to the chorda-sheath. N medullary tube, Ch chorda,
Lh body-cavity, A atrium, L upper wall of same, E1 inner wall, E2
outer wall, Lh1 ventral remnant of same, Kst gill-reds, M ventral
muscles, R seam of the joining of the ventral folds (gill-covers), G
sexual glands.)
Above the sexual glands, at the dorsal angle of the atrium, we find the
kidneys. These important excretory organs could not be found in the
Amphioxus for a long time, on account of their remote position and
their smallness; they were discovered in 1890 by Theodor Boveri
(Figure 2.217 x). They are short segmented canals; corresponding to
the primitive kidneys of the other vertebrates (Figure 2.218 B). Their
internal aperture (Figure 2.217 B) opens into the body-cavity; their

outer aperture into the atrium (C). The prorenal canals lie in the middle
of the line of the head, outwards from the uppermost section of the
gill-arches, and have important relations to the branchial vessels (H).
For this reason, and in their whole arrangement, the primitive kidneys
of the Amphioxus show clearly that they are equivalent to the prorenal
canals of the Craniotes (Figure 2.218 B). The prorenal duct of the latter
(Figure 2.218 C) corresponds to the branchial cavity or atrium of the
former (Figure 2.217 C).
(FIGURE 2.217. Transverse section through the middle of the
Amphioxus. (From Boveri.) On the left a gill-rod has been struck, and
on the right a gill-cleft; consequently on the left we see the whole of a
prorenal canal (x), on the right only the section of its fore-leg. A genital
chamber (ventral section of the gonocoel), x pronephridium, B its
coelom-aperture, C atrium, D body-cavity, E visceral cavity, F
subintestinal vein, G aorta (the left branch connected by a branchial
vessel with the subintestinal vein), H renal vessel.
FIGURE 2.218. Transverse section of a primitive fish embryo
(Selachii-embryo, from Boveri.). To the left pronephridia (B), the right
primitive kidneys (A). The dotted lines on the right indicate the later
opening of the primitive kidney canals (A) into the prorenal duct (C). D
body-cavity, E visceral cavity, F subintestinal vein, G aorta, H renal
vessel.)
If we sum up the results of our anatomic study of the Amphioxus, and
compare them with the familiar organisation of man, we shall find an
immense distance between the two. As a fact, the highest summit of the
vertebrate organisation which man represents is in every respect so far
above the lowest stage, at which the lancelet remains, that one would at
first scarcely believe it possible to class both animals in the same
division of the animal kingdom. Nevertheless, this classification is
indisputably just. Man is only a more advanced stage of the vertebral
type that we find unmistakably in the Amphioxus in its characteristic
features. We need only recall the picture of the ideal Primitive
Vertebrate given in a former chapter, and compare it with the lower
stages of human embryonic development, to convince ourselves of our
close relationship to the lancelet. (Cf.
Chapter 1.

11.)
It is true that the Amphioxus is far below all other living vertebrates. It
is true that it has no separate head, no developed brain or skull, the
characteristic feature of the other vertebrates. It is (probably as a result
of degeneration) without the auscultory organ and the centralised heart
that all the others have; and it has no fully-formed kidneys. Every
single organ in it is simpler and less advanced than in any of the others.
Yet the characteristic connection and arrangement of all the organs is
just the same as in the other vertebrates. All these, moreover, pass,
during their embryonic development, through a stage in which their
whole organisation is no higher than that of the Amphioxus, but is
substantially identical with it.
(FIGURE 2.219. Transverse section of the head of the Amphioxus (at
the limit of the first and second third of the body). (From Boveri) a
aorta (here double), b atrium, c chorda, co umlaut coeloma
(body-cavity), e endostyl (hypobranchial groove), g gonads (ovaries),
kb gill-arches, kd