CHAPTER DC 



THE VERTEBRATES: THE AMPHIBIA AND BIRDS 



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 pecidiarly suited 

 for comparative study. Moreover, the rather large size of many vertebrate 

 eggs has made them favourite objects for experimental analysis, and our 

 understanding of the epigenetics of tlie 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 ob- 

 jects, 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 favour- 

 able 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 turn to consider, more shortly, the embryology of the other vertebrate 

 phyla. 



Detailed descriptions of the development of Amphibia are to be found 

 in many general embryological textbooks, particularly Dalcq and Gerard 

 (1935)- For the chick there are several special monographs. Patten (1950) 

 and Huettner (1949) are good descriptive texts, the figures in the latter 

 being particularly clear; Hamilton's (1952) revision of Lillie is the most 

 complete descriptive monograph, but tends to neglect non-American 

 work; Waddington (1952^) deals mainly with the early stages of develop- 

 ment, and with experimental studies. Details of most microsurgical tech- 

 niques are given by Hamburger (1942) and Rugh (1948), but neither of 

 them deals with organiser grafts in birds. 



I. From the unfertilised egg to the formation of the blastula 

 [a) The Amphibia 



The fully grown frog's or newt's egg is a fairly large spherical cell, some 

 2 or 3 mm. in diameter. It is usually seen after being extruded into the 



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