114 EMBEYOLOGY OF THE LOWER VERTEBRATES ch. 



archaic groups of Vertebrates (Elasmobranchii and Dipnoi) the motor 

 nerve-trunk is already present as a protoplasmic bridge at a time 

 when myotome and spinal cord have not yet commenced to recede 

 from one another. It does not seem possible to explain the appear- 

 ances recorded in these cases by any conceivable errors of observa- 

 tion. But if such bridges exist in these relatively archaic groups, 

 the balance of probability is entirely on the side of their represent- 

 ing the primitive mode of development of nerve-trunks in general, 

 and of a fundamentally similar mode of development occurring 

 in other Vertebrates though possibly in a modified and less distinct 

 form. 



On the other hand appearances of the kind which led to the 

 original formulation of the His view, and which are still adduced in 

 its support, and which are easily observed in series of sections 

 through almost any type pf Vertebrate embryo — nerve -trunks 

 passing out from the spinal cord and ending freely amongst the 

 mesenchyme — are peculiarly apt to be misleading. 



Such a misleading appearance is produced sometimes by compara- 

 tively simple causes — by breakage of the nerve-trunk or by the 

 nerve-trunk passing away out of the plane of a section and being 

 unrecognizable when cut transversely in a neighbouring section. In 

 other cases the appearance of a freely ending nerve-trunk is due to 

 the portion of nerve-trunk which has received its protoplasmic sheath 

 being distinctly visible in a stained section, while the delicate peri- 

 pheral portion which is still naked is practically unrecognizable. On 

 account of such liability to misinterpretation a very large proportion 

 of the observational evidence which supports the His view is open to 

 suspicion. 



A physiological difficulty which has been raised against accepting 

 the His view is that involved in the idea that the free end of the 

 growing fibre tracks down and finds its appropriate end-organ. It 

 is pointed out that it never makes a mistake — never becomes joined 

 up to a wrong end cell. And yet, if it be the case that nerve-fibres 

 do grow outwards with free ends in the way involved in the His 

 theory, certain experimental results show that such fibres do possess 

 a very decided power of making mistakes. This is brought out 

 clearly by the beautiful experiments of Braus (1905). 



In the experiments in question Braus made use of the method, 

 invented by Born and developed by Harrison, Spemann and others, 

 in which portions of one amphibian embryo are grafted upon the 

 body of another, when the grafted portion (" parasite " — Braus) pro- 

 ceeds to develop as part of the individual (" autosite " — Braus) upon 

 which it has been grafted. 



In the experiments which are most important in their bearing on 

 the point now under discussion the early rudiment of the pectoral 

 limb was grafted upon a host in the region of the head. In this 

 position the rudiment went on developing into a perfectly normal 

 limb containing a normal arrangement of the limb nerves. Now the 



