PREFACE 



A number OF teachers of embryology have expressed the desire to see 

 experimental studies incorporated into the material that is presented to begin- 

 ning students of the subject. This book is the result of an attempt to bring 

 together observations and experiments into a unified exposition. In order to 

 do this within a reasonable compass, the description of development has been 

 limited to essentials. In the laboratory with the aid of an adequate laboratory 

 manual the student can discover for himself the detailed anatomy of various 

 embryos. 



Embryology is a dynamic science par excellence. It is the study of a 

 gradual change from one stage to another. The changes are very slow, but 

 they are continuous. It does not suffice to stop at any one point to study the 

 anatomy of the embryo, because these stopping points are arbitrary. Embry- 

 ology should never be simply a description of the anatomy of the embryo. 

 Nor can it be merely comparative. The details of comparative and descrip- 

 tive embryology are of specialized interest and are of use chiefly to the 

 specialist. The attempt here is to achieve a free-running, continuous treatment 

 of the principles and theories of development. 



The theories are supported by various experiments and various observa- 

 tions. Another teacher will, no doubt, use many other experiments and 

 observations to supplement and replace these specific experiments. He will 

 also want to criticize certain interpretations and point out to the student that 

 the problems in this field, like those in many other fields, are not finally 

 settled, and that many theories and hypotheses are only first approximations 

 to the truth. But in spite of all these criticisms and in spite of experimental 

 errors, the principles survive, and the emphasis must remain on principles. 



In order to keep the continuity of the text unbroken, references to books 



