GASTRULATION PROPER 



117 



Dorsal side 



Ventral side 



i'?/Yolk 

 plug 



Blastopore lip 



Ectoderm 



esoderm 



Endoderm 



W-Yolk plug 



H 

 Ventral blastopore lip 



Archenteron 



Mesenchyme 



Notochord Medullary plate 



Dorsal blasto- 

 pore lip 



Midgut 



Gastral mesoderm 



Foregut 

 Mesenchyme 



Notochord 

 Hindgut 



Blastopore 



Liver diverticulum 



Gastral mesoderm 



Blastula to gastrula stages in the frog — (Continued) Shown by stereograms. 

 (Modified and redrawn from Huettner.) 



The forces which bring about the initial involution or inturning of 

 cells at a specific point of the marginal zone to form the dorsal lip 

 of the blastopore have not yet been identified. The nearest approach 

 to a valid explanation is that of Holtfreter. He says: 



The alkalinity of the blastocoel can be assumed to be strong enough to 

 establish, in cooperation with the suspended protein particles, an efficient 

 gradient of surface tension between the internal and external medium, as 

 envisaged by Rhumbler. Those cells which are in interfacial contact with 

 both media would be primarily affected by it. They will tend to move in the 

 direction of the surface tension lowering alkaline medium, and to reduce 

 their contact area with the external medium having a higher surface tension. 

 Being, however, attached to the peripheral coat they can only stretch along 

 the gradient, assuming a shape which can be expected to correspond to those 

 claviform cells which we find in all cases where invagination takes place. 

 The peipendicular stretching will have to continue as long as the gradient 



