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 emibryological 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 



