The authors of the eleven chapters in this book approach the subject from a variety of viewpoints, ranging from an analysis of molecular changes within differentiating cells to a consideration of the factors governing the transplantation of normal and abnormal tissues. The book is based on papers presented at the Twelfth Growth Symposium of the Society for the Study of Development and Growth. Originally published in 1955. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.