634 MR W. E. AGAR ON DEVELOPMENT OF ANTERIOR MESODERM, AND 



of the anterior member of the compound larvae quickly extends backwards over the 

 grafted-on tail, owing to the method of growth of the ectoderm by general proliferation, 

 and of the myotomes by apical growth. This relative shifting is extremely great. 

 Thus (p. 441) " . . .a given (ectoderm) cell located originally at the posterior limit of 

 the fourteenth myotome moves during seven days to a point at least 2*5 mm. further 

 peripherally (posteriorly), that is, through a distance equal to about fourteen segments." 



This movement of a given ectodermal cell backwards over the underlying myotomes 

 was found by Harrison to be much greater the further back the cell was situated, 

 owing to the fact that at the time of the experiment the tail region was growing very 

 much more rapidly than the anterior region. He found by experiment, however, that 

 there was a slight movement of the ectoderm in the same direction in the region of the 

 pronephros and fore-limb. Still at a much earlier stage, when the trunk itself and the 

 anterior portion of the tail are lengthening out more rapidly, it is extremely probable 

 that a backward movement of any given point of the ectoderm over the underlying 

 myotomes takes place to a nearly equal extent. 



If the initial change in the ectoderm really acts as the stimulus to the development 

 of the mesodermal portion of the fin rudiment, the bearing of Harrison's experiment 

 on the question of limb migration is obvious. Owing to the different method of growth 

 of the ectoderm and mesoderm, and to the development of the anterior part of the body 

 before the posterior part, any given point in the ectoderm, at first at the level of about, 

 say, the first metotic myotome, will later on be far down the trunk. Hence the ecto- 

 dermal rudiment of a structure originally at the hinder head region will tend to appear 

 further back. If now the completion of the structure follows, by the self- regulative 

 processes with which we are so familiar now, on the stimulus of this initial ectoderm 

 rudiment — even when this is displaced from its original position (like the optic vesicle 

 in Lewis's experiment) — we have, as a pure result of this different method of growth of 

 ectoderm and mesoderm, an effective cause of the backward migration of such posterior 

 head structures into the trunk. 



Of course this is not meant to supply a complete explanation of the present position 

 of the limbs. It only shows how the mode of development of the vertebrate body may 

 lead to a strong tendency to produce that backward movement of posterior head 

 structures (postulated by GJ-egenbaur's theory of the origin of the paired limbs) which 

 has afforded material for natural selection to work upon in fixing the position of the 

 limbs. The recent slight forward migration of the pectoral fin in Elasmobranchs 

 (Bratjs) and Dipnoi must in any case be regarded as an entirely secondary acquirement. 



I put this forward as a mere suggestion. I am well aware that without some firmer 

 grounds to go upon than we have at present it is not worthy of consideration as a 

 practical hypothesis. Nevertheless, I thought that if this were borne in mind, no harm 

 would be done by recording the idea. Meanwhile we may consider one or two points 

 which have bearings upon the question. 



Different forms give different answers to the question as to whether the ectoderm 



