CHEMICAL NATURE OF THE ORGANIZER 85 



Chemical nature of the organizer 



What do we know about the chemical nature of this stimulus? Investi- 

 gators have found that a number of tissues from the adult, such as liver or 

 kidney or pancreas, will stimulate the ectoderm to become a neural tube. 

 Thus other tissues imitate to some extent at least the action of the organizer. 

 These adult tissues do not have to be alive in order to induce a neural tube. 

 In fact, many of them induce larger and better formed neural tubes after 

 they are killed or fixed. Something present in tissues in general is able to 

 stimulate the neural plate to differentiate. This widespread occurrence of 

 inducing substances encouraged embryologists to test a number of relatively 

 pure chemical compounds. And it was soon found that a variety of chemical 

 compounds, such as the fatty acids, sterols, protein extracts — especially 

 nucleoproteins — and some synthetic carcinogenic agents — agents producing 

 tumors or cancers — when applied to the ectoderm of the early gastrula would 

 cause this ectoderm to form a neural tube. 



Figure 39 shows a longitudinal section of an early gastrula. Chemical 

 compounds are tested for organizer activity by introduction into the blasto- 

 coele of the early gastrula. If the chemicals are suspended in some substance 

 such as agar or egg albumen, which form solids, and the mixtures are intro- 

 duced into the blastocoele, the chemical implants will come in contact with 

 the presumptive epidermis. During gastrulation and subsequent development 

 the substances will come to lie in the ventral body region. And in this region 

 there will be an outgrowth containing a well-defined neural tube. This 

 neural tube develops from ectoderm which has been in contact with the 

 chemical implant. One of the best chemical inductors is the tobacco mosaic 

 virus, which is a pure nucleoprotein. 



These chemical stimulants do not imitate the organizer action completely. 

 They do not convert the ectoderm into a secondary embryo, as does the 

 organizer action which we outlined in the preceding chapter. Pure chemical 

 compounds simply induce neural tubes. Some investigators have therefore 

 assumed that, in addition to a primary chemical stimulus, the organizer region 

 may contain other compounds, which are sometimes called modulators. In 

 other words, the organizer is thought to contain two kinds of substances: an 

 inductor substance for the neural tube and, in addition, a second factor which 

 changes this neural tube into the specific parts of the nervous system — brain, 

 spinal cord, etc. 



