648 STRUCTURE AND GROWTH OF DOMESTICATED ANIMALS. 
different classes in the higher animals. And if we pass from the 
class of fishes to the lower types of the animal kingdom,—to 
insects, for instance, crustacea, and worms,—we find everywhere 
the same process. Even the parasitic intestinal worms are now 
known to be produced by eggs, and eggs which are transferred by 
various processes from one animal to another, sometimes with 
their food or drink, at other times by boring into the body of their 
host, thus remaining parasites in succeeding generations. The 
same thing has been observed among the various kinds of mollusks, 
—the cuttlefish and periwinkles, the oysters, mussels, etc., for 
all these produce eggs; and when the eggs are examined, at the 
proper time, and in a proper manner, they exhibit exactly the same 
structure as those of the higher classes; and we may go down to 
the very lowest classes of the animal kingdom — the seaurchins, 
the starfish, the jellyfish, or even the corals or polypes, and there 
again eggs are found, and eggs which in no way differ from those 
of the higher animals. 
From such statements, whieh cover now such extensive ground, 
it might be inferred that to know one is equal to knowing all. By 
no means ; for enough has already been done to show us that every 
one has its peculiarities, every one has its own mode of develop- 
ment, and in every one there are peculiar processes which make 
the generalization only true in the most comprehensive form of 
expression, and no longer true in the details of the farther devel- 
opment. So that all our knowledge of the process of reproduc- 
tion in one species of animals may not give us an answer when 
we would inquire into the corresponding process in another ani- 
mal. us you see the necessity of repeating for those animals, 
the breeding of which we would desire to influence, all those o 
servations which have been made upon a few. 
I should like presently to make some remarks as to the kind of 
training necessary for this, that you may not imagine that the 
first enthusiast can go to work and do it. It requires a long 
training to be prepared to look at an egg, to be prepared to see 
how it grows; but before I make any such remarks, I would say 
a few words more concerning the formation of the germ, so that 
you may see what an interesting field of observation is now open 
to the student; open, not yet cultivated; by no means cultivated 
to the extent desirable in order to make the knowledge in any 
way useful in practical life. There is that condition necessary tO 
