In this influential work, Frank Rattray Lillie explores the process of adaptation in embryonic development. This volume focuses specifically on the cleavage period, when the zygote undergoes a series of rapid cell divisions. With detailed illustrations and clear explanations of experimental methods, this book is a seminal text in the field of developmental biology. This work has been selected by scholars as being culturally important, and is part of the knowledge base of civilization as we know it. This work is in the "public domain in the United States of America, and possibly other nations. Within the United States, you may freely copy and distribute this work, as no entity (individual or corporate) has a copyright on the body of the work. Scholars believe, and we concur, that this work is important enough to be preserved, reproduced, and made generally available to the public. We appreciate your support of the preservation process, and thank you for being an important part of keeping this knowledge alive and relevant.