REPRODUCTION OF PRIMITIVE NERVE CELLS 87 



irreversibly changed into a nerve cell. And when this embryonic nerve cell 

 divides — and this is important — it gives rise to two cells of the same type. 

 We must assume, therefore, that the liberated substance reproduces itself 

 between cell divisions (Fig. 40) . That is, once a substance is liberated within 

 the ectoderm cell and the substance causes it to differentiate into an embryonic 

 nerve cell, the substance responsible for the differentiation must reproduce 

 itself inside the cell between cell divisions. We shall return to this aspect of 

 differentiation later. 



The fact that some carcinogenic agents stimulate ectoderm to form a 

 neural tube suggests a similarity between the differentiation of cells in the 

 embryo and the formation of cancer cells in the adult. In both cases new 

 types of cells arise by chemical stimulus. If the skin of a rabbit is painted with 

 carcinogenic agents, tumors will form. If the ectoderm of the early gastrula 

 is treated with some of the same agents, a neural tube will form. Since 

 embryonic cells have many potencies, it is not surprising that carcinogenic 

 stimuli will cause them to differentiate into new types of cells. But adult cells 

 have usually been thought to have lost this ability to change. However, if we 

 consider the origin of tumors and cancers as simply a change from a normal 

 cell to a new type of cell, then adult cells must still have some potencies which 

 are usually not expressed. 



Reproduction of primitive nerve cells 



The problem of the differentiation of any cell into a new type of cell 

 carries with it a second problem. The new type of cell must reproduce itself 

 after the initial change has occurred. That is, after the stimulus has been 

 applied and removed, the cell must go on reproducing its own kind. The 

 changes which have been brought about during differentiation must be 

 inherited in some way. How can this be visualized ? It does not seem likely 

 that there has been a change in the nucleus or the nuclear genes of the cell, 

 since the genetic make-ups of different types of cells in the body are regarded 

 as identical. The nuclear genes in the cells from the nervous system are 

 regarded as identical with those in muscle. Therefore some inherited change 

 in the cytoplasm must account for the reproduction of muscle and nerve cells. 



Recent studies have conclusively established cytoplasmic inheritance as a 

 fact. Certain elements in the cytoplasm can reproduce themselves in the same 

 manner as genes in the nucleus. The exact mechanism of this cytoplasmic 



