220 PRINCIPLES OF ANIMAL BIOLOGY 



are known. In general, however, the events of early embryonic develop- 

 ment appear to be more or less independent, though working in harmony. 

 Probably they are helped to keep in the proper order of time by organizers 

 that successively arise. 



Nature of Organizers. — These organizers are not specific, not effec- 

 tive merely in their own species, since transplants between species show 

 about the same consequences as those within species. This fact has 

 encouraged a search for the nature of such influences, for the same ones 

 must be fairly general and widespread. Almost certainly organizers are 

 chemical substances. A number of organic acids have been shown to 

 induce certain differentiations. Among them are several of the fatty 

 acids, nucleic acid, and adenylic acid from muscle. There is some 

 indication that the sterols (higher alcohols) have inductive powers. 

 Glycogen is probably in some way connected with the power of induction 

 in salamanders, for while the cells are being rolled over the rim of the 

 blastopore during gastrulation (which is about the time at which these 

 cells first acquire the power to induce nervous system), they rapidly 

 lose their glycogen. What an organizer does to stimulate development, 

 what happens between the stimulus and the response in differentiation, 

 is unknown. 



The power of an organizer to induce a certain event usually lasts much 

 longer than there is any need of it in ordinary development. Thus, 

 notochord and mesoderm, taken from embryos in which nervous systems 

 have long since been irrevocably established above them, are still capable 

 of stimulating secondary nervous systems in younger embryos into which 

 they are transplanted. The power of the tissues to respond to organizers 

 is, however, not so persistent. Usually they must be stimulated at about 

 a certain time, or they cannot respond at all. 



In general, it may be said that the inherent properties of the tissues 

 to respond by developing arc more important than the stimuli received 

 from organizers. 



References 



Bailey, F. R., and A. M. Miller. Textbook of Embryology. William Wood & 



Company. (Chaps. I-VI.) 

 Hegner, 11. W. The Germ Cell Cycle in Animals. The Macniillan Company. 



(Chaps. I and II.) 

 Holmes, S. J. The Biology of the Frog. The Macmillan Conii)aiiy. (Chap. V.) 

 Kellicott, W. E. a Textbook of General Embryology. Henry Holt & Company, 



Inc. (('haps. VI-VIII, cleavage to formation of germ layers.) 

 Morgan, T. H. The Development of the Frog's Egg. The Macmillan Company. 



(Chap. V, early development.) 

 Morgan, T. H. Experimental Embryology. Columbia University Press. (Chap. 



XV, fate decided before cleavage; ("laps. XVII and XVIII, partial embryos; 



Chap. XIX, fate determined by position.) 



