8 PRINCIPLES OF EMBRYOLOGY 



organs which require radical alteration before the adult appears. There are, 

 of course, too many types of adult or larva in the whole animal kingdom 

 for it to be possible to give a single scheme of basic organs which can 

 apply to them all, but it is perhaps worth while to indicate the general 

 pattern of all the various types of vertebrates. Such a scheme is shown 

 in Fig. I.I. We see that the ectoderm forms, firstly, the skin which covers 

 the whole body, and secondly a thickened plate which folds up to form 

 first a groove and fmally a tube which sinks below the surface and differ- 

 entiates into the central nervous system. (At the boundary between the 

 neural and skin parts of the ectoderm, cells leave the ectodermal sheet and 

 move into the interior of the embryo; this 'neural crest', which forms 

 nervous ganglia and other organs is not shown in the Figure.) The sheet 

 of mesoderm becomes split up longitudinally into a series of zones. Under 

 the midline of the embryo is a long rod-like structure, the notochord, 

 which is the first skeletal element to appear. On each side of this the meso- 

 derm is thickened and transversely segmented so that it takes the form of a 

 series of roughly cuboidal blocks, which are knovra. as somites, and which 

 give rise to the main muscles of the trunk as well as the inner layers of the 

 skin. Laterally on each side of the somites there is a zone of mesoderm 

 which wiU later produce the nephroi or kidneys, and laterally again more 

 mesoderm which is not transversely segmented and which is destined to 

 give rise to the limbs and the more ventral muscles and sub-epidermal 

 skin. Finally, in the most inner recesses of the embryo, the endoderm 

 becomes folded into a tubular structure which is the beginning of the gut 

 or intestine. The formation of these organs always begins earher in the 

 anterior end of the embryo than in the posterior. 



After these basic elements in the adult structure have been roughed out, 

 there remains, of course, much to be done in adding the details, but the 

 phenomena differ so much in the various phyla that there is no point in 

 trying to describe more stages of general application. 



3. Phylogenetic theories of embryology 



Until fairly recently, the main theoretical concern of embryologists has 

 been to find a guiding principle which would allow them to arrange 

 the enormous mass of descriptions of developmental changes into some 

 sort of orderly whole. The chief such principle was found in the theory 

 of evolution. Long before Darwin, at a time when the idea of evolution 

 was little more than a nebulous speculation, Meckel suggested (about 

 1 8 10) that a developing embryo of a 'higher' form of animal passes 

 through a series of stages which represent the adults of the 'lower' forms 

 ancestral to it. For instance at one stage the embryo bird has gill-slits, 



