440 PRINCIPLES OF EMBRYOLOGY 



most substances than are the membranes bounding the inner surfaces of 

 the cells (Fig. 20.10). 



There seems Httle doubt that expansions and contractions of the coat 

 are important factors in early morphogenesis. Holtfreter pointed out 

 that if unfertiHsed eggs are kept in media of different osmotic pressure 

 for several days, the pigmented coat of the animal half tends to expand 

 in a manner which reminds one of the expansion of the animal pole 

 region of the blastula during gastrulation. Indeed, after treatment with 

 Ringer solution, the expanded animal coat may dive into the interior of 

 the egg in a way which quite closely simulates the process of invagination. 

 This suggests that the coat has some inherent capacity to carry out such 



Figure 20.10 



Fragment of a morula of the axolotl, showing the pigmented surface coat. 



(From Holtfreter I943-) 



movements, independently of the cellularisation of the underlying mater- 

 ial. It would be interesting to know whether the expansion of the coat is 

 connected with the formation of any oriented fibrous micro-structure 

 within it. The fact that the expansion is greater in the one plane, in which 

 the 'invagination' takes place, suggests that this is the case, but there has 

 been no direct demonstration of it owing to the difficulty of polarised 

 light studies on such material. 



In stages later than gastrulation the coat may also have important mor- 

 phogenetic effects. If small groups of cell are placed in an air/hquid or 

 hquid/hquid interface, the cell membranes will be disrupted if the surface 

 tension is powerful enough. By using a number of interfaces of appro- 

 priate surface tensions, Waddington (i942<:) showed that the effective 

 strength of the coated surfaces of cells from gastrula and neurula stages 

 gradually mcreases as development proceeds, and that as the neural plate 



