This book should be regarded as the continuation to my previous book Developmental Biology of the Sea Urchin Embryo, edited by the Academic Press in 1973, rather than as a new edition. Due to the exceedingly high rate of development in this field (something like 2000 papers have been published on this subject in these last 10 years), I preferred, in fact, not to describe again in detail the enormous amount of the old literature, as was attempted in my previous book, but to briefly summarize the state of the art in each problem and to describe in some detail the experiments per­ formed in the last 12 years. In doing so, more emphasis was given to the more recent ones and to those which can be considered as corner stones in each subject. Care was, however, taken to mention the reviews or key papers in which the reader can find a source of the details of the older literature, besides refering him to my previous book.