218 PRINCIPLES OF ANIMAL BIOLOGY 



of animals there is no fundamental difference between the cells of different 

 regions. For example, the cells of a sea urchin embryo, in the two- or 

 four- or eight-cell stage, may be separated from one another, and each 

 becomes a complete, though small, larva. If left in contact with the 

 other cells, each cell would have produced only certain parts of a single 

 animal, but it obviously has the capacity to produce all of it. In a few 

 animals, however, the cells are in some respect different, for, if the cleav- 

 age cells are separated, each one gives rise only to a fraction of a larva. 

 Animals of the former type are said to have indetermininate development, 

 the latter kind determinate development. 



The cells of indeterminate embryos take on their specific destinies 

 at a mut;h later time. This has been most completely shown for some 

 of the salamanders. If, at a time shortly after gastrulation begins, bits 

 of tissue are transposed, a group of cells that would normally become 

 nervous system exchanging places with a group that would become epi- 

 dermis, the fate of each is altered. The would-be part of a nervous 

 system becomes epidermis, the prospective epidermis becomes nervous 

 system. The exchange of regenerating stumps of fore- and hind limbs, 

 described in the preceding section, is a similar example. The interchange 

 of bits of tissue may be made between different species ^vith equal success. 

 One such interspecific exchange was effected between species differing 

 in color, one very light-colored, the other quite dark. The cells retained 

 their color characters but produced strange organs. In one experiment, 

 presumptive brain cells of a dark species were transplanted to the region 

 on a light species where gills develop. Now these species differ not only 

 in color; their gills are of different shapes. The transplanted dark cells, 

 while being converted into gills instead of brain, produced gills of the 

 form characteristic of the dark species. The general fate of the cells 

 may be altered, but their specific performance within the general field 

 remains unchanged. 



In all these examples the fate of the transplanted tissues had not 

 yet been determined. For each of them, however, there comes a time 

 after which such reversals of fate are no longer possible. After that time, 

 transplanted parts become what they would have become in their original 

 situation. If, for example, a patch of ectoderm including a portion of the 

 neural folds (a stage shortly after the end of gastrulation) is placed on the 

 side of the body, it Ijecomes nervous system despite its strange location. 

 Something has hapi)ened to these cells during the process of gastrulation 

 wliich has deprived them of the capacity to respond to their position in 

 the embryo and has fixed their fate regardless of location. An area of 

 such determined ectoderm may oven be cut out of the embryo and culti- 

 vated by itself in a suitable salt solution, and it still develops the sort of 

 organ (nervous system, for example) which it was destined to become. 



