482 APPENDIX 



shown in figs. 213, 214, It is of interest to note that the tendency to 

 constriction in the marginal zone (Ch. iii, p. 42) still manifests 

 itself, leading to a waist between the ectoderm and the endo- 

 mesoderm from the earliest stages of gastrulation. Later the waist 

 becomes still further narrowed to a stalk, which can be easily 

 severed, and may break of its own accord. 



All the mass-movements of the different regions involved in 

 normal gastrulation (p. 43) are still operative in the exogastrulae, 

 though their mutual interactions in the altered circumstances are a 

 little different, as indicated by the arrows in figs. 213, 215. For 



^^^^ 



r' 



Fig. 214 

 Exogastrulated axolotl embryo, 8 days old. On the right, the epidermis ; on the 

 left, the exogastrulated endo-mesoderm : note pharynx with inverted gill- 

 pouches (on left). (From Holtfreter, Biol. Zentralbl. liii, 1933.) 



instance, the organiser-region, forming the dorsal side of the 

 marginal zone (p. 41), stretches out as a tongue on the dorsal side 

 of the evaginated mass, and subsequently becomes sunk in a 

 groove and finally overgrown by the endoderm. This confirms the 

 view that the dynamics of gastrulation are predetermined in the 

 various local regions of the germ. 



The final result of exogastrulation is what we may call an exo- 

 embryo (Holtfreter's Exokeim). This consists of two very distinct 

 parts. The ectoderm has flattened down to an irregular wrinkled 

 mass with the blastocoel largely obliterated. It shows no medullary 

 differentiations, notably no trace of neural tube or even of local 

 thickening to form neural plate tissue. 



