A DESCRIPTIVE SKETCH 2$ 



differing from the Urodele picn in certain details of relative life 

 of regions, the Anuran plan is fundamentally similar (figs. 5 and 6). 

 The process of gastrulation entails remarkable streaming move- 

 ments and displacements of the various regions of the embryo. 

 Presumptive notochord, mesodermal somites, and mesoderm of 

 the lateral plate become tucked in over the rim of the blastopore 

 lip as already described. Their places on the surface of the embryo 

 are taken by the presumptive neural fold region undergoing dis- 

 placement, its original position being occupied by the expanding 

 region of the presumptive epidermis. This displacement and ex- 

 pansion, however, takes place in a peculiar way. It must be re- 

 membered that at the start of gastrulation the lip of the blastopore 

 is present only in the dorsal meridian, and the lateral lips are formed 

 later. Consequently, the material on the dorsal meridian becomes 

 tucked in first and reaches further forward on the under-side of the 

 superficial layer of the embryo than does material which is situated 

 more laterally. One result of this state of affairs has already been 

 noted : the piling up of the yolk-cells on the ventral side of the gut- 

 cavity with the resultant rotation of the whole embryo to conform 

 to the new centre of gravity. There is another important result: 

 since the disappearance of the cells from the surface of the embryo 

 and their plunging in over the lip of the blastopore is more active 

 on the dorsal side, there is a consequent stretching of the regions 

 right and left of the dorsal meridian, and a movement towards that 

 meridian to take the place of the invaginated material. In this way, 

 the two horns of the crescent-shaped region of the presumptive 

 neural folds, which at the start of gastrulation were situated at the 

 sides of the embryo, now move nearer to the dorsal mid-line and 

 to one another, so that they form parallel strips which eventually 

 enclose the blastopore between their hindmost ends. Between 

 these parallel strips, the central part of the presumptive neural fold 

 region stretches backwards along the dorsal meridian to the dorsal 

 lip of the blastopore, which it reaches. Thus, instead of lying as a 

 transverse band across the embryo as at the blastula stage, the pre- 

 sumptive neural fold region after gastrulation occupies a position 

 extending longitudinally along the dorsal side of the embryo, where 

 the neural folds will in fact arise.^ It may be referred to at this 



^ Goerttler. 1925. 



