Let us now take a glance at certain stages of embryonic development which correspond to these earliest ancestral forms.  We should expect some such correspondence from the fact already stated that the embryonic development of the individual is a brief recapitulation of the ancestral development of the species or larger group.  The egg of the lowest vertebrate, amphioxus, shows these changes in a simple and apparently primitive form.

  [Illustration:  3.  Immature egg-shell from ovary of echinoderm. 
  HATSCHEK, from Hertwig.]

The fertilized egg of any animal consists of a single cell, a little mass of protoplasm containing a nucleus and surrounded by a structureless membrane.  The egg is globular.  The nucleus undergoes certain very peculiar, still but little understood, changes and divides into two.  The protoplasm also soon divides into two masses clustering each around its own nucleus.  The plane of division will be marked around the outside by a circular furrow, but the cells will still remain united by a large part of the membrane which bounds their adjacent, newly formed, internal faces.

Let us suppose that the egg lay so that the first plane of division was vertical and extending north and south.  Each cell or half of the egg will divide into two precisely as before.  The new plane of division will be vertical, but extending east and west.  Each plane passes through the centre of the egg, and the four cells are of the same form and size, like much-rounded quarters of an orange.  The third plane will lie horizontal or equatorial, and will divide each of these quarters into an upper and lower octant.  The cells keep on dividing rapidly, the eight form sixteen, then thirty-two, etc.  The sharp angle by which the cells met at the centre has become rounded off, and has left a little space, the segmentation cavity, filled with fluid in the middle of the embryo.  The cells continue to press or be crowded away from the centre and form a layer one cell deep on the surface of the sphere.

This embryo, resembling a hollow rubber ball filled with fluid, is called a blastosphere.  It corresponds in structure with the fully developed volvox, except, of course, in lacking reproductive cells.

  [Illustration:  4.  Gastrula.  HATSCHEK, from Hertwig. 
  Outer layer is the ectoderm; inner layer, the entoderm; internal
  cavity, the archenteron; mouth of cavity, blastopore.]

If the rubber ball has a hole in it so that I can squeeze out the water, I can thrust the one-half into the other, and change the ball into a double-walled cup.  A similar change takes place in the embryo.  The cells of the lower half of the blastosphere are slightly larger than those of the upper half.  This lower hemisphere flattens and then thrusts itself, or is invaginated, into the upper hemisphere of smaller