ebrate relationships
already appear. Figure 7 ii. is a cross-section, its position, being
shown by cross-lines in 7 i. and 6. Note first that the epiblast along
the mid-dorsal line is sinking in to form what is called the neural
plate (n.p.), and simultaneously on either side of it rise the neural
folds (n.f.). Now, at Figure 8, a slightly later stage is represented, and
at 9 i. the inturned part is separated from the general external epiblast
as the spinal cord. The remainder of the epiblast constitutes the
epidermis.

Section 19. Reverting to Figure 7 ii., along the dorsal side of the
archenteron a thickening of its wall appears, and is gradually pinched
off from it to form a cellular rod, lying along under the nervous axis and
above the intestine. This is the notochord (compare Figures 8 and 9).

Section 20. Finally, we note two series of buds of cells, one on either
side of the archenteron in Figure 7 ii. In 8 these buds have become
hollow vesicles, growing out from it, the coelomic pouches. They are
further developed in 9; and in 9 ii., which is a diagrammatic figure,
they are indicated by dotted lines. They finally appear to (? entirely)
obliterate the segmentation cavity-- they certainly do so throughout
the body; and their cavities are in time cut off from the mesenteron, by
the gradual constriction of their openings. In this way the coelom
(body cavity) arises as a series of hollow "archenteric" outgrowths,
and ms. becomes the alimentary canal. mt.c., the metapleural
canals, probably arise subsequently to, and independently of, the
general coelomic space, by a splitting in the body-wall substance.

Section 21. Hence, in considering the structure of amphioxus, we
have three series of cells from which its tissues are developed:--

1. The epiblast.

2. Walls of the coelomic pouches, which form (a) an inner lining
to the epiblast, (b) an outer coating to the hypoblast, and (c) the
mesentery (m.), by which the intestine is supported. This is the
mesoblast.

3. The lining of the mesenteron, or hypoblast.

From the epiblast the epidermis (not the dermis), the nervous system
(including the nerves), and the sensory part of all sense organs are
derived. From the mesoblast the muscles, the dermis genital and
excretory organs, circulatory fluid and apparatus, any skeletal
structures; and all connective tissue are derived. The mass of the
body is thus evidently made of mesoblast. The hypoblas