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PRINCIPLES OF EMBRYOLOGY 



dorsal midline became longer from anterior to posterior and at the same 

 time narrower from side to side, while in more lateral regions these two 

 changes in dimensions "were less marked. Indeed, on the ventral side 

 there would have to be some increase in the side-to-side dimension to 

 compensate for the narrowing that occurs near the dorsal plane. This 

 locally-variable change in dimensions constitutes the first factor to be 

 accounted for. The second is the fact that the dorsal material, as it in- 

 creases its anterior-posterior length, is tucked inside the blastocoele to 

 form a primitive gut instead of merely protruding as a process sticking 

 out from the region of the blastopore. 



Epidermis 



Head 



mesodbem 



Figure 20.12 



A, B are drawings of two stages of exo-gastrulation in the axolotl; the ecto- 

 derm lies above, and the endo-mesoderm, instead of moving inside it, is 

 elongating towards the bottom of the picture. C is a diagrammatic section 

 through a later stage. (After Holtfrcter 1933.) 



The second factor has attracted considerably more study than the first. 

 It can fairly easily go wrong if embryos, particularly of the axolotl, are 

 allowed to gastrulate in solutions of abnormal osmotic pressure. There is 

 then often a failure of the mechanism by which the elongating dorsal 

 material should be turned inwards, and part of the mesoderm and endo- 

 derm (or even the whole of it) may move outwards from the blastopore, 

 forming a so-called exogastrula. In less strongly affected cases it is the 

 anterior mesoderm which fails to get inside, while the more posterior 



