A DESCRIPTIVE SKETCH 21 



The conditions in the Anuran type during gastrulation are in the 

 main similar to those in the Urodele, except that, for reasons into 

 which we need not here enter, the gut-cavity possesses its definitive 

 endodermal gut-roof from the start. This definitive roof is com- 

 plete except for a thin longitudinal strip corresponding to the noto- 

 chord and to the cells immediately underlying it which will give rise 

 to the hypochordal rod. When the notochord and hypochordal rod 

 become lifted off from the gut-roof, a narrow gap is formed, but it 

 soon becomes closed by the approximation of the free edges of the 

 endoderm.^ 



§4 



Since the cells that become tucked in during gastrulation were 

 originally on the outer surface of the blastula before the process of 

 gastrulation started, it is possible to outline on the surface of the 

 blastula the various regions which will, in normal development, 

 give rise to the various organs of the future embryo. By the 

 method alluded to above, of making stains intra vitam in particular 

 places on the surface of the blastula, and by following their changes 

 of position during gastrulation and subsequent development, it is 

 possible to discover the normal futures in store for all the regions 

 of the blastula, and in this manner to ascertain their normal 

 potencies. One may thus speak of the various regions of the blastula 

 as presumptive organs: one region is presumptive notochord, 

 another presumptive brain, and so forth. 



By methods of this kind, and by making small injuries in definite 

 places w^ith the electric cautery, Vogt and his pupils have been able 

 to map the amphibian blastula completely in terms of presumptive 

 organ-rudiments. This has been accomplished both for a Urodele 

 and an Anuran type.- 



For purposes of description, a system of notation similar to that 

 used in fixing the positions of places on the earth's surface will be 

 found convenient. The dorsal meridian of the egg or blastula, 

 which passes through the future dorsal lip of the blastopore, may 

 be taken as a standard meridian, corresponding to the meridian of 

 Greenwich in geography, and other meridians may be indicated 

 by degrees of longitude, right or left, from the dorsal meridian. 

 ^ Mayer, 193 1. ^ Vogt, 1929; Suzuki, 1928. 



