534 - Heredity and Evolution 



half an embryo. But in most cases a deter- 

 mination of the cells comes much later in 

 the embryonic period. 



In the frog embryo during gastrulation 

 (p. 277), small groups of cells may be trans- 

 planted from almost any part of the embryo 

 and induced to grow in various new positions 

 in the same embryo. Such transplanted cells 

 usually change their destiny and develop into 

 structures that are normal for the region to 

 which they have been moved. But if similar 

 transplantations are made at a later stage, 

 the results are altogether different. Now the 

 transplanted cells give rise to structures char- 

 acteristic of the region from which they have 

 been moved. The cells of a limb bud, for 

 example, moved from the shoulder region to 

 the side of the abdomen, now cling to their 

 original destiny, giving rise to a forelimb 

 that is quite normal except for the peculi- 

 arity of its position. At some time between 



gastrulation and the appearance of the limb 

 bud, these cells have lost their totipotency; 

 that is, they have become unipotent, or deter- 

 mined, as to their eventual fate, or destina- 

 tion. 



Embryonic Organizers. One group of cells in 

 the frog embryo becomes determined much 

 earlier than the others. During gastrulation 

 the cells constituting the dorsal lip of the 

 blastopore (Fig. 27-7), if transplanted to an- 

 other part of another embryo, go on develop- 

 ing in the same manner as they would have 

 done had they not been moved. Moreover, 

 the transplanted dorsal lip not only deter- 

 mines its own development, but it also organ- 

 izes the development of the neighboring 

 cells. Soon an extra neural tube begins to 

 form in the host embryo, in front of the site 

 of the transplanted dorsal lip (Fig. 27-8); 

 and an extra vertebral column, together with 

 associated structures, is induced to form in 



UNFERTILIZED EGG 



AFTER FERTILIZATION 



BLASTULA 



DORSAL 

 UP 



DORSAL- 

 LIP 



DORSAL 



LIP / / 

 YOLK PLUG Z VENTRAL LIP 



GASTRULATION 



Fig. 27-7. Early development of a frog embryo. The primary axis of the un- 

 fertilized egg is vertical; i.e., the animal pole is on top. After fertilization, the 

 gray crescent (left) appears. The secondary axis is now horizontal and the 

 median plane corresponds with the surface of the page. During gastrulation the 

 whole embryo rotates as indicated by the arrow, so that the primary axis 

 originally vertical, becomes the longitudinal axis, with the animal pole at the 

 anterior end; the secondary axis, originally horizontal, becomes the dorsoventral 

 axis, with the original gray crescent region on the dorsal side. 



