APPENDIX 487 



bud arises in a more or less typical way; but it never becomes 

 large, its growth is soon arrested, and it is finally resorbed, in spite 

 of the absence of any degenerative signs in its tissues. Holtfreter 

 ascribes this (and also the absence of regular segmental arrange- 

 ment of the trunk-muscles) to the absence of the neural tube and 

 especially to the absence of the mesenchyme derived from the neural 

 crest, which is known (see pp. 193, 396) to have the tendency for 

 directive outgrowth. As the neural tube and crest are first in- 

 duced, by the chorda-mesoderm, and then supply material necessary 

 for tail-mesoderm diflferentiation, we have here an interesting case of 

 mutual induction on the part of an organiser and of that which it 

 organises. The same interaction is apparent in the head-region, 

 where neural crest material, originally induced by the prechordal 

 portion of the organiser, appears to be necessary for the proper 

 anatomical differentiation of the tissues (muscle and cartilage) 

 derived from this region. (See also p. 181 for a comparable case of 

 mutual dependence in sea-urchins.) 



Among other special points may be mentioned the fact that teeth, 

 even partial or rudimentary, are never found in exo-embryos, 

 showing that the presence of ectoderm is necessary for their initia- 

 tion. Taste-buds, however, do differentiate in the pharynx, thus 

 demonstrating that the view sometimes maintained of their deriva- 

 tion from immigrant ectoderm is incorrect (see p . 498). The presence 

 of gill-clefts shows that their initial determination proceeds from 

 the pharyngeal ectoderm and is quite independent of the presence 

 of ectoderm. The fact that they later disappear, however, suggests 

 that contact with ectoderm is needed for their maintenance. 



Blood-tissue is rarely found in exo-embryos, apparently because 

 its primary site of origin lies far back in the ventro-caudal region, 

 and from here it often tends to become included within the 

 ectodermic vesicle. 



The ciliary beat on the surface of the ectoderm is also of interest. 

 In the normal embryo this is directed in an orderly way, in a 

 predominantly antero-posterior direction (see p. 236). In wholly 

 isolated ectodermic vesicles, however, it is completely irregular, 

 indicating that a polarity or polarized gradient-field is normally 

 imposed upon the epidermis from the underlying endo-mesodermal 

 tissues. This is beautifully demonstrated by cases in which exo- 



