126 POPULAR SCIENCE MONTHLY. 



fault of embryology, the tendency to explain any and every opera- 

 tion of development as merely the result of inheritance.' It has been 

 demonstrated that surrounding conditions have much to do with in- 

 dividual development, and that the course of events may depend 

 largely upon stimuli coming from without, and not exclusively on an 

 inherited tendency. 



Cell-Lineage. — Investigations on the structural side have reached 

 a high grade of perfection in studies on cell-lineage. The theoretical 

 conclusions embodied in the germ-layer theory are based upon the 

 assumption of identity in origin of the different layers. But the lack 

 of agreement among observers, especially in reference to the origin of 

 the mesoderm, made it necessary to study more closely the early de- 

 velopmental 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 fol- 

 lowed, 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 work of Whitman, Wilson, Conklin, Kofoid, Lillie, Mead 

 and Castle has 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 this theory is retained in a much 

 modified form. 



Theoretical Discussions. — Certain theoretical discussions, based on 

 embryological studies, have been rife in recent years. And it is to 

 be recognized without question, that discussions regarding heredit37 

 regeneration, the nature of the developmental process, the question of 

 inherited organization within the egg, or 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, furnish us with a knowledge of living forms 

 along lines of structure, function and development. The embryo- 

 logical method of study is of increasing importance to comparative 

 anatomy and physiology. Formerly it was entirely structural, but is 

 now becoming, also, experimental, and will be of more service to 

 physiology. While it has a strictly technical side, the science of 

 embryology must always remain of interest to intelligent people as 

 embracing one of the most wonderful processes in nature — the de- 

 velopment of a complex organism from the single-celled condition, 

 with a panoramic representation of all intermediate stages. 



