488 APPENDIX 



gastrulation is not complete, but a portion of the ectoderm has been 

 underlain by endo-mesoderm and has been organised. In such 

 portions, the direction of ciliary beat is regular and normal, while 

 remaining irregular over the rest of the epidermis (fig. 218). 



This brings us to a more general consideration of partial exo- 

 gastrulation. Total exogastrulation is a comparatively rare occur- 



I 



s. 



Fig. 218 



Induction of polarity in epidermis by underlying organiser. The direction of 

 cilia-beat (indicated by arrows) of non-underlain epidermis is irregular and 

 chaotic; that of epidermis underlain by organiser-tissue is regular and polarised. 

 (From Holtfreter, Biol. Zentralbl. Liii, 1933.) 



rence : in the majority of cases, exogastrulation only proceeds to a 

 certain point, and then the remainder of the endo-mesoderm is 

 invaginated under the ectoderm. All gradations are to be found 

 from a minimal invagination to a normal embryo. The first step is 

 the presence of some blood- and yolk-cells in the ectodermic 

 vesicle. When they are present, they induce a smooth two-layered 

 epithelium, with normal tempo of differentiation, in place of the 

 irregularly folded and wrinkled epidermis derived from wholly 



