is formed behind in the same way a few hours later (in the vicinity of
the additional gastrula-mouth). In man and the higher vertebrates also
the mouth and anus are formed, as we have seen, as flat pits in the
outer skin; they then penetrate inwards, gradually becoming connected
with the blind ends of the closed gut-tube. During the second day the
Amphioxus-embryo undergoes few other changes. The number of primitive
segments increases, and generally amounts to fourteen, some
forty-eight to fifty hours after impregnation.

Almost simultaneously with the formation of the mouth the first
gill-cleft breaks through in the fore section of the Amphioxus-embryo
(generally forty hours after the commencement of development). It now
begins to nourish itself independently, as the food material stored up
in the ovum is completely used up. The further development of the free
larvae takes place very slowly, and extends over several months. The
body becomes much longer, and is compressed at the sides, the head-end
being broadened in a sort of triangle. Two rudimentary sense-organs
are developed in it. Inside we find the first blood-vessels, an upper
or dorsal vessel, corresponding to the aorta, between the gut and the
dorsal cord, and a lower or ventral vessel, corresponding to the
subintestinal vein, at the lower border of the gut. Now, the gills or
respiratory organs also are formed at the fore-end of the alimentary
canal. The whole of the anterior or respiratory section of the gut is
converted into a gill-crate, which is pierced trellis-wise by numbers
of branchial-holes, as in the ascidia. This is done by the foremost
part of the gut-wall joining star-wise with the outer skin, and the
formation of clefts at the point of connection, piercing the wall and
leading into the gut from without. At first there are very few of
these branchial clefts; but there are soon a number of them--first in
one, then in two, rows. The foremost gill-cleft is the oldest. In the