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consist of single cells of protoplasm, but differ in this respect 
that the animal-cells have no limiting membrane as is the rule 
with the protophytes. 'flic mode of reproduction is, practically, 
identical with that described for the lowest plants, and, therefore, 
I need only say that it consists in enlargement by growth of the 
parent-cell and a subsequent constricting off of a smaller or larger 
portion of its substance, whereby a new individual is produced. 
To illustrate the mode of development from the single cell of 
the higher animals, I shall select the common chicken, since I am 
able to present specimens of the living embryo at different stages 
of incubation. 
The origin of the chick is from an extremely minute unit of 
protoplasm, looking, at first, as seen in the parent-bird, like the 
cells which are to develope into muscle or nerve, or any other 
tissue, but soon becoming slightly larger, and sufficient !) 7 modified 
to indicate its higher destiny. After a time this little cell is sur¬ 
rounded by an enormous quantity of nutritive material, intended 
to support its growth during the period of incubation, and this 
whole mass we term an egg.* When the time has come for the 
development of this germ-cell, its protoplasm divides into two 
halves, each of these into two again, making four, and so on, until 
we have spread out upon a small area of the nutritive mass of the 
egg a layer of germinal or embryonic cells, constituting by their 
aggregation a membrane.f This membrane constantly enlarging 
in every direction, by the multiplication of its constituent cells, 
soon shows a cleft of separation in its mass, after which it con¬ 
tinues to grow as two portions, although these remain attached at 
the site of their earliest growth ; in other words, the embryonic 
cells are now arranged in two parallel layers or membranes, 
which correspond to the two seed-leaves, or cotyledons, of the 
plant-embryo within the seed. Later, an intermediate membrane 
is put forth by the embryonic mass between the first two, and, at 
a still later stage, by a rift in this third layer it is converted into 
two, and so the embryo is found to consist of four membranes. 
By the subsequent thickening and folding of these four layers, in 
various ways, the whole body of the chick is finally built up # 
*Fig. io. tFig" JI * 
