June 11, 1909] 



SCIENCE 



933 



confuse the student seriously. His confusion 

 is increased if he compares different books, 

 for they are apt to select different illustrations 

 from the unlimited body of facts; and state- 

 ments are often contradictory. Moreover, it 

 is frequently impossible to say 'when the de- 

 scription passes from one form to another, or 

 indeed, what species of animal, or pale phan- 

 tom of the imagination, is in the author's 

 mind at all. 



The distinction between fact and theory is 

 a useful one, even in embryology; indeed, I 

 know of no distinction so important for the 

 student to master. But in the text-books of 

 embryology there is a nebulous zone between 

 peopled with morulse, blastulae, gastrulse, 

 germ-layers, etc., which dissolve and reappear 

 in strange forms never the same. And the 

 student is often uncertain whether he is on 

 the sure ground of fact, in the fascinating 

 field of theory or in the twilight zone between. 

 Such uncertainty is demoralizing, because it 

 limits his respect for exact facts and does not 

 increase his capacity for sound generalization. 

 The student who gains his conception of a 

 developmental sequence by combining the 

 morula of a mammal, the blastula of a starfish, 

 the gastrula of AmpMoxus and the germ- 

 layers of a frog is as far removed from con- 

 nected facts as from soimd theory. 



Such method of the text-book is of course 

 ill-adapted to laboratory practise and there is 

 iisually a gap between. The student's labora- 

 tory practise is usually limited to an anatom- 

 ical study of a few stages of one or a few 

 forms, and if he turns to the text-book to bind 

 his observations into a sequence, he finds brief 

 superficial descriptions of disconnected stages 

 of a great variety of forms, for the most part, 

 of course, different from those he is studying 

 in the laboratory. Neither in his laboratory 

 practise nor in his text-book does the student 

 obtain genuine understanding of the develop- 

 ment of any single form. It would be just as 

 reasonable to expect proper comprehension of 

 the principles of comparative anatomy of ver- 

 tebrates without knowledge of the anatomy of 

 any definite species as to expect real under- 

 standing of the principles of comparative em- 



bryology from a student who does not know a 

 single life history thoroughly. 



These conditions are seriously aggravated 

 by secondary considerations of the text-books, 

 such as the alleged greater significance of 

 certain aspects of the life history for various 

 practical disciplines, which leads to unjust 

 emphasis, and to the view that embryology is 

 a field from which only facts of practical 

 value are to be culled; I am far from denying 

 the significance of the study of embryology 

 for medicine, for instance, but I would main- 

 tain that much of its significance is lost by 

 piecemeal selections. If the facts and prin- 

 ciples are understood, the applications may be 

 readily made at the proper time and place, but 

 if they are not understood the applications are 

 surely of doubtful value. 



Theory changes so rapidly in embryology 

 that text-books soon grow out of date; and 

 tastes differ so widely that the selection of 

 facts for a book fails to satisfy more than a 

 limited number of teachers. Hence the con- 

 stant procession of text-books of embryology. 

 In this state of affairs there is bound to be a 

 reaction, and it appears to me that this must 

 take the form of a series of text-books dealing 

 with the concrete development of single forms. 



Probably no text-book of embryology has 

 been so infiuential and of so long continued 

 service as Foster and Balfour's " Elements of 

 Embryology," of which the first edition was 

 put forth in 1873, and the second edition 

 (enlarged) in 1883, soon after Balfour's un- 

 timely death. Even now, although it has not 

 been revised for twenty-six years, it is still in 

 active service. The reason for this lies partly 

 in the simplicity and clearness of the style, 

 but largely in the fact that the greater part 

 is a literal account of the development of a 

 single form, the chick; it takes up the events 

 of the development of each day, to the end of 

 the sixth day at least, " as though the devel- 

 opment were done by day labor," which is 

 indeed the case, and the student obtains some 

 idea of time-relations which are of the essence 

 of embryonic development; he is made to see 

 development as a continuous process marching 

 steadily forward to a definite consummation; 



