A Community of Cells 11 



effected by the proteins present in the chromosome and it may be that 

 after a certain number of subdivisions a gene becomes bare and there- 

 fore effective. But this is all very speculative and at the present time 

 we can only be amazed at the precision with which the process of de- 

 velopment occurs. Not only are cells produced which have distinct 

 functions, but shape and form is accurately reproduced. The fingers 

 grow at similar rates until the adult hand is formed. The characteristic 

 features of the face are even passed down from one generation to 

 another. We have to admit our almost complete ignorance of the 

 controlling factors. 



The ability of tissues to produce cells of differentiated types is 

 greatest in the early stages of the development of the embryo and is 

 lost as the completed organism takes shape. In fact, in the very early 

 embryo, the future destiny of the cells seems to be largely undeter- 

 mined. H. Spemann studied the early stages of development of the 

 newt, before the stage known as gastrulation was reached. When the 

 developing embryos were cut in two, a whole embryo developed 

 from each half. However, this possibility is lost very quickly, and dif- 

 ferent parts are formed which have destinies (e.g. to form heart tissue, 

 or intestine) which cannot be altered. Indeed these 'committed' cells 

 are able to modify and impress their destiny on any other tissue im- 

 planted in them. It is necessary to suppose that effects of this kind are 

 brought about by chemical substances, called organizers, but very 

 little is known as to their nature. If we 'transplant a young and undif- 

 ferentiated cell group into the region of the head,' says Paul Weiss.^ 

 *it will form eye or brain; transplant it to the anterior trunk and it will 

 form limb; or further back, kidney; and transplant it to the rear and 

 it will form tail — the same cells forming different structures depend- 

 ing on the location. We may say: organizing factors take hold of the 

 cells and direct them to appropriate functions.' 



However, we know little about these organizing factors. Indeed, 

 many biologists see more in an organism than a collection of cells. 

 The cells all fit into each other and make a whole. If the whole is dis- 

 rupted, it is reconstructed. 'Each cell develops in conformity with its 

 surroundings. ... It becomes a cone or a rod when in the retina, a 

 cartilage cell when in the centre of the limb bud, a neurone when in 

 the brain. Intrinsically capable of a variety of performances the cell 

 receives some definite cue from the locality indicating which trend it 

 is to follow. These cues are decidedly of supracellular origin.' 



It follows from this that we must regard the organism as more than 

 a mere collection of cells. It undoubtedly has a unity of its own, which 

 in some ways impresses its character and needs on the individual 



^ American Naturalist, 24, 43, 1946. 



