APPENDIX 



483 



The endo-mesoderm on the other hand bears a considerable 

 resemblance to an embryo, showing well-marked regions — a head 

 and gill-region with gill-clefts, a trunk-region, and a tail-region. 

 However, it is entirely abnormal in its detailed structure. It is 

 morphologically inside-out; its outer 

 layer is endodermal, and this contains 

 a more or less solid mass of notochord, 

 somites, mesenchyme and cartilage 

 (figs. 213, 216). 



The endodermal epithelium is, as 

 in the normal embryo, polarised: but 

 its outer surface corresponds with that 

 which bounds the gut-lumen in normal 

 ontogeny. This may be compared with 

 the fact noted on p. 250, that spheres 

 composed of gastral layer only (collar- 

 cells), arising in sponge dissociation 

 experiments, have the collars directed 

 outwards, whereas in normal animals 

 they face the gastral cavity. In both 

 sponges and amphibia, one surface of 

 the epithelium orients itself towards 

 the most favourable environment, 

 whether this be an internal lumen or 

 the external medium. 



Exogastrulation gives us a method 

 by which ectoderm can be totally 

 separated from endo-mesoderm from 

 the first onset of the gastrulation- 

 process. In addition, it provides pseudo- 

 embryos, containing all the derivatives 



of the endo-mesoderm, in which we can be certain that no nervous 

 tissue is present ; and further, in the inversion of the endodermal 

 and mesodermal layers, it provides a natural experiment in ab- 

 normal spatial relations which it would be impossible to duplicate 

 artificially. (See also p. 252 for a comparable case in insects.) 



As might be expected, conclusions of considerable importance 

 have been arrived at by analysis of the results. In the first place, we 



31-2 



Fig. 215 



Diagrams of transverse sec- 

 tions through Urodele em- 

 bryos, showing the structure 

 and directions of movement 

 of parts in a, normal embryo, 

 h, exo-embryo. v.P. site of 

 original vegetative pole. (From 

 Holtfreter, Arch. Entwmech. 

 cxxix, 1933.) 



