THE RISE OF EMBRYOLOGY 235 
cially in reference to the origin of the mesoderm, made it 
necessary to study more closely the early developmental stages 
before the establishment of the germ-layers. It is a great 
triumph of exact observation that, although continually 
changing, the consecutive history of the individual cells has 
been followed from the beginning of segmentation to the time 
when the germ-layers are established. Some of the beautifully 
illustrated memoirs in this field are highly artistic. 
Blochman (1882) was a pioneer in observations of this 
kind, and, following him, a number of American investigators 
have pursued studies on cell-lineage with great success. 
The researches of Whitman, Wilson, Conklin, Kofoid, Lillie, 
Mead, and Castle have given us the history of the origin of 
the germ-layers, cell by cell, in a variety of animal forms. 
These studies have shown that there is a lack of uniformity 
in the origin of at least the middle layer, and therefore 
there can be no strict homology of its derivatives. This 
makes it apparent that the earlier generalizations of the 
germ-layer theory were too sweeping, and, as a result, the 
theory is retained in a much modified form. 
Theoretical Discussions.—Certain theoretical discus- 
sions, based on embryological studies, have been rife in recent 
years. And it is to be recognized without question that dis- 
cussions regarding heredity, regeneration, the nature of the 
developmental process, the question of inherited organiza- 
tion within the egg, of germinal continuity, etc., have done 
much to advance the subject of embryology. 
Embryology is one of the three great departments of 
biology which, taken in combination, supply us with a knowl- 
edge of living forms along lines of structure, function, and 
development. The embryological method of study is of in- 
creasing importance to comparative anatomy and physiology. 
Formerly it was entirely structural, but it is now becoming 
also experimental, and it will therefore be of more service to 
