the left
auricle. Then the left ventricle, aortic arches (for the future pulmonary
artery is in communication by a part presently blocked, the ductus
arterious, with the systemic aorta), arteries, capillaries, veins. The
liver capillary system and the pulmonary system only become
inserted upon the circulation at a comparatively late stage.

Section 34. With the exception of the reduction of the pronephros,
what has been said of the development of the frog's nervous system,
renal and reproductive organs, and skeleton, applies sufficiently to the
fowl for our present purposes. The entire separation of Wolffian and
Mullerian ducts from the very beginning of development is here beyond
all question (vide Section 18). But the notochord in the fowl is not so
distinctly connected with the hypoblast, and so distinct from the
mesoblast, as it is in the lower type, and no gills, internal or external,
are ever developed. The gill slits occur with a modification due to the
slitting and flattening out of the embryo, already insisted upon; for,
whereas in the tadpole they may be described as perforations, in the
fowl they appear as four notches between ingrowing processes that
are endeavouring to meet in the middle line.

_The Development of the Rabbit_

Section 35. The early development of the rabbit is apt to puzzle
students a little at first. We have an ovum practically free from yolk
(alecithal), and, therefore, we find it dividing completely and almost
equally. We naturally assume, from what we have learnt, that the next
stages will be the formation of a hollow blastosphere, invagination, a
gastrula forming mesoblast by hollow outgrowths from the
archenteron, and so on. There is no yolk here to substitute epiboly
(Section 9) for invagination, nor to obliterate the archenteron and the
blastopore through its pressure.

Yet none of these things we have anticipated occur!

We find solid mesoblastic somites, we find primitive streak, allantois
and amnion, features we have just been explaining as the
consequence of an excess of yolk in the egg. We even find a yolk sac
with no yolk in it.

Section 36. A solid mass of cells is formed at the beginning, called a
morula, Figure 1. In this we are able to distinguish rather smaller
outer layer cells (o.l.c.), and rather larger inner layer cells (i.l.c.),
but these cells, in their later development, do not answer at all to the
two primitive layers of the gastrula, and the name o