490 APPENDIX 



is invaginated, the actual neural plate may be much narrower than 

 the presumptive neural region (see p. 155). Further, it does not 

 differentiate into a complete but undersized nervous system, but 

 into tail neural tube only : the organiser, in other words, has regional 

 properties. If only a slight degree of invagination occurs, the in- 

 vaginated material is presumptive caudal tissue, and the induced 

 structure is a tail (fig. 220 a). This then grows out as quite a normal 



Partial exogastrulation and progressive organising effects of varying degrees of 

 organiser-invagination. a, slight invagination (of tail-organiser only) producing 

 only a tail ; b, medium invagination (of tail- and trunk-organiser) producing tail 

 and trunk ending anteriorly with gills ; c, nearly complete invagination (tail-, 

 trunk-, and head-organiser) producing an embryo which is complete except 

 for the diminutive length of the head and size of the eyes. (From Holtfreter, 

 Biol. Zentralbl. liii, 1933.) 



tail, confirming the conclusions reached above (p. 193) as to the 

 co-operation of mesoderm and neural crest mesenchyme in normal 

 tail-elongation. With progressive increase in the amount of tissue 

 invaginated, there is a progressive increase in the amount of 

 organisation of the ectoderm, first trunk-structures appearing, then 

 gills, and finally head-structures (fig. 220 b and c). The direction of 

 ciliary beat is normal on the organised portions of the ectoderm of 

 such partial embryos (e.g. with tail only), irregular on the un- 



