Morphogenesis 581 



as well as in the regeneration of lost parts, there must be an inherent 

 blueprint which is followed if the structure is to develop typically. Of 

 what docs this blueprint or master plan consist? Since all living or- 

 ganisms, according to the cell principle, are composed of cells, it would 

 be surmised that cells form the basis for this origin and development. 

 Evidence is available that this is true, but a cell is a complex structural 

 unit composed of many integrated, component parts. Which parts 

 specifically guide this remarkable phenomenon of living organisms? 

 Experimental evidence, at least in certain organisms, suggests that the 

 cytoplasm as well as the nucleus may play an important role in this con- 

 nection. Experiments on the eggs of certain echinoderms suggest that 

 cytoplasm from which all formal, organized nuclear material has been 

 separated is capable of originating and developing the embryo up to a 

 certain stage. Probably, beyond this stage the nuclear materials (genes, 

 etc.) take over and influence the specific traits which develop. It is 

 known that genes are units capable of self-duplication within the living 

 cells, although their multiplication outside the living cell has not been 

 observed to date. Since genes are known determiners of hereditary traits 

 and are capable of multiplication in living cells, we have an explanation 

 for the development of similar cells during the process of cell division. 



However, a multicellular organism which arose from a single cell 

 (zygote) is composed of thousands of cells, but the cells are not, and 

 cannot, all be alike. Certain kinds must be differentiated (developed 

 differently) so that the various types of tissues and organs may be 

 formed. What forces control this differentiation? What changes occur 

 in the organization of the protoplasm whereby different structures may 

 arise from what was originally the same material? At present, scientists 

 do not know the complete answer to this question. However, by the 

 application of the scientific method in the study of these problems, bit 

 by bit additional information is being secured. It is known that the 

 abilities of a cell or tissue during emBryologic development are influenced 

 by (1) the inherent, intrinsic abilities of that cell or tissue and (2) the 

 environmental forces around it. 



By careful, scientific studies of the development of frog embryos, the 

 embryologist Spemann found that each of the cells (of the two-celled 

 stage) when completely separated by a fine hair loop would develop into 

 a normal, although small, tadpole. What does this mean? Simply 

 that each separated cell of the original pair is capable of forming a com- 

 plete, diminutive individual whose genes are necessarily like those of 

 its mechanically divorced partner. If the two cells had not been arti- 



