BIOLOGY IN HUMAN AFFAIRS 



general ground plan of the body, perhaps the common 

 characteristics of the order or the family, is due to the 

 mother alone. Another plausible conclusion can also be 

 drawn from these facts. Since if one of the cytoplasmic 

 layers of the sea urchin is removed by a tiny pipette, 

 the part of the body to which it should give rise does 

 not develop, it appears to follow that the nucleus can 

 perform its governmental function but once in a given cell 

 generation. 



Another important group of internal environmental 

 factors may be termed factors of position. With favorable 

 material, such as the sea urchin, the two cells resulting 

 from the first division of the fertilized egg (or even the 

 four-celled and eight-celled stages of later divisions) may 

 be shaken apart. Each cell then produces a normal sur- 

 face layer over the area where it has previously been in 

 contact, and restores the respiratory conditions necessary 

 to, and characteristic of, the normal egg. The two cells, 

 which would have produced the right and left halves of a 

 single individual, now proceed to develop into complete 

 organisms. With other material, separation of the cells 

 results in the production of right- and left-half organisms. 

 With still other material — the frog is a good example — 

 the egg itself may be cut into two parts; and each part will 

 produce a half individual. What is the cause of these 

 peculiar results? The answer given by the biologists best 

 able to judge is this: Organisms vary somewhat in the 

 detailed maneuvers by which they differentiate; but in 

 every case development is a process of sorting out the 

 various cytoplasmic materials and is directed by the genes 

 under certain restrictions offered by cell position. 



The conclusion that the course of development taken by 

 any cell lineage depends upon its relations to other cells 

 is not a speculation or an indirect inference; it is a direct 

 conclusion based on experimental evidence. In the early 



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