432 THE MAINTENANCE OF SPECIES 



head develops. Such differences in metaboHc rates between differ- 

 ent parts of an organism have been demonstrated experimentally 

 and it is probable that they are related to differences in the oxygen 

 supply. 



Natural Potencies 



Great differences normally occur between the so-called "potencies" 

 of various species of eggs. Some species of animals produce toti- 

 potent eggs. These are eggs in which the formative material is equally 

 distributed throughout the component cells, or hlastomeres during 

 early development. The resulting cleavage is called indeterminate 

 because all cells up to a certain stage are totipotent, a condition 

 that may be demonstrated by separating the various blastomeres, 

 for example, from the two-celled to the sixteen-celled stage in some 

 of the jellyfish, and securing normal, though perhaps dwarfed, indi- 

 viduals from each. Cleavage in man is apparently of this type, and 

 is the logical explanation of the production of identical twins. 



In the case of non-totipotent species the cleavage pattern is said to 

 be determinate. There is little doubt that many of the determinative 

 factors are already present in the cytoplasm of an egg before it is 

 fertilized. In such forms as the mollusc, Dentalium, or the tunicate, 

 Styela, the cytoplasm of the egg itself appears to be arranged in a 

 definite pattern with respect to its future development. In such 

 cases the early separation of blastomeres results in the formation of 

 partial embryos. 



Organizers 



Certain parts of embryos are called organizers because they appear 

 to be more or less directly responsible for the development of other 

 closely associated regions. Much experimental work has been done 

 abroad by Spemann and his co-workers, and in this country by 

 Harrison and his students, all of which demonstrates the presence of 

 such organizers. Perhaps one of the most important organizers is the 

 dorsal lip of the blastopore. That this region is normally associated 

 with the development of a neural plate may be demonstrated by 

 transplanting it to a region beneath the ventral ectoderm of a frog's 

 gastrula, where one would normally expect the formation of epidermis, 

 but instead an aberrant neural plate appears. Such experimental 

 evidence has been most carefully checked and rechecked by all manner 



