ABSTRACT
THE GROUP of vertebrates contains a variety of types which are sufficiently diverse to exhibit most of the important principles of embryology, but are not so bewilderingly various as to obscure the fundamental plan of which they are all modifications. They are therefore peculiarly suited for comparative study. Moreover, the rather large size of many vertebrate eggs has made them favourite objects for experimental analysis, and our wlderstanding of the epigenetics of the group is at least as great as that of any of the invertebrate phyla. This is particularly true of the Amphibia, and only slightly less so of the birds. These two groups have for long been classical teaching material, since frogs' and chickens' eggs are some of the easiest to obtain for students' use. The discussion of vertebrate development in this book will be largely based on these same two objects, although as an amphibian type, the newt's egg will be referred to perhaps more often than that of the frog, since, although they are basically similar in the characters of interest in an elementary account, the former shows these features in a somewhat clearer way; moreover, for technical reasons connected with ease of manipulation, it has proved more favourable than the frog egg for experimentation. It is only after the early development of these two types has been described and discussed that we shall tum to consider, more shortly, the embryology of the other vertebrate phyla.
