486 APPENDIX 



ment occurs at a corresponding stage, though of course the cells 

 here are detached into the surrounding medium (fig. 217 ^ and c). 

 The process occurs only in the central region of the gut corre- 

 sponding to the small intestine. This determination of a timed 

 degeneration recalls that of the isolated chick mesonephros (p. 205). 



The attainment of functional activity by many tissues in the 

 demonstrable total absence of all nervous tissue is of great interest 

 (cf. p. 430). The epidermis, gut, and pronephros tissue, and prob- 

 ably thyroid vesicles, embark upon active secretion, and ciliary 

 activity sets in where expected. Spontaneous rhythmic movements 

 of the outward-facing gut-endothelium occur regularly, brought 

 about by the underlying smooth musculature, thus providing the 

 first demonstration of the independence of this tissue of innerv- 

 ation for its functional differentiation. The heart may also reach 

 this stage, confirming explanation experiments (p. 203). The 

 striated skeletal musculature, however, was never observed to 

 contract, either spontaneously, or in reaction to mechanical, 

 chemical or electrical stimuli: later, the degenerative changes 

 typical of denervated striped muscle set in. Thus the histological 

 difl^erentiation of skeletal muscle can be reached (though not 

 maintained) in the total absence of nervous connexions, but not its 

 functional activity. This confirms and extends other work (p. 431). 



In spite of the remarkable self-diflFerentiating powers of the endo- 

 mesoderm, the structure of the exo-embryo is abnormal in a 

 number of respects. The head and trunk-musculature, though 

 differentiating histologically into typical striated fibres, is not 

 arranged in a regular metameric plan, and the direction of the fibres 

 is irregular. The cartilages of the head (no cartilage appears to be 

 formed in the trunk-region) are massed together in a single irregular 

 lump. The total absence of the cartilages arising from the neural 

 crest (p. 394) is doubtless largely responsible for the lack of regular 

 arrangement of the head-musculature, as well as for its small size. 

 The gonad appears not as a long ridge, but as a series of small cell- 

 masses in close connexion with the pronephric tubules. The liver 

 tissue is always very small in amount, and late in appearance ; no 

 gall-bladder has been noted. 



The tail-region is of particular interest. A caudal zone of activity 

 is present in notochord and trunk-musculature, and a conical tail- 



