STRUCTURE AND GROWTH OF DOMESTICATED ANIMALS. 643 
posed to be widely different one from another, both in structure 
and in mode of reproduction; but less than fifty years ago, a 
German physiologist, Karl Ernst von Baer, one of the ablest 
investigators of our century, made the astounding discovery that 
all animals bring forth eggs that may not be distinguished from 
one another at a certain stage; that all our cattle, all our domes- 
ticated animals, all the beasts of the forest, as well as all the 
birds on earth, produce eggs similar to one another. This seems 
a very extraordinary statement, yet perhaps I shall be able to 
make you familiar with the fact, and to make you understand it 
as fully as you know that your hens lay eggs. But the eggs of a 
great many animals most useful to us, and of those about which 
we would like to know most, have not been studied microscop- 
ically. Ihave devoted a great deal of my life to similar topics, 
and I have never yet seen the egg of a mare; I have never yet 
seen the egg of a cow; I have never yet seen the egg of a pig; 
yet I believe that these animals bring forth eggs as much as the 
animals that have been investigated with reference to that point. 
A sufficient number of quadrupeds have been studied to leave no 
doubt that all quadrupeds produce eggs as well as birds, as well as 
all other animals, without exception. One of the ablest physiolo- 
gists of our time, Professor Bischoff, of Munich, has devoted over 
twenty years of his life to the study of a few of these animals, 
and the results of his investigations are embodied in a volume of 
many hundreds of pages, with a large number of plates, represent- 
ing the history of only four species of quadrupeds. One is the 
rabbit, another is the dog, a third is the guinea-pig, and the fourth 
a species of deer which is common in the forests of Europe, — the 
roebuck ; and the history of these animals, as presented in this 
volume, covers only the very earliest period of gestation,—and 
mainly that portion of their history embraced during the first 
days of gestation, during the time when the egg of these animals 
is transformed into a germ which grows to be an animal like the 
parent. Now, unless we can have a similar history of any one of 
our more valuable domesticated animals, as of the horse, or of the 
cow, we cannot expect to know how to influence their reproduc- 
tion. This is the very foundation of all knowledge in that 
direction. What will be necessary for that? When these inves- 
tigations began they were ER upon animals which could be 
Secured at the lowest price; they were begun with the hen. Two 
