THE ORGANIZATION CENTER 399 



same stage of development, is able to produce a secondary gastrulative process 

 and thus initiate the formation of a secondary embryo (fig. 193). Because 

 the dorsal-lip tissue was able thus to organize the development of a second 

 or twin embryo, Spemann and Mangold described the dorsal-lip region of the 

 beginning gastrula as an "organizer" of the gastrulative process. In its normal 

 position during gastrulation this area of cells has since been regarded as the 

 organization center of amphibian development. It is to be observed in this 

 connection that Lewis ('07) performed the same type of experiment but failed 

 to use an embryo of the same age as a host. As he used an older embryo, the 

 notochordal and mesodermal cells developed according to their presumptive 

 fate into notochordal and somitic tissue but failed to organize a new embryo. 



More recent experiments upon early frog embryos by Vintemberger ('36) 

 and by Dalcq and Pasteels ('37), and upon early teleost fish embryos by other 

 investigators (Oppenheimer, '36 and '47) have demonstrated the necessity 

 and importance of yolk substance in the gastrulative process. This fact led 

 Dalcq and Pasteels ('37) to suggest a new concept of the organization center, 

 namely, that this center is dependent upon two factors: "the yolk and some- 

 thing normally bound to the gray crescent" (i.e., chordamesodermal area). 



It was thought at first that the transplanted organizer material actually or- 

 ganized and produced the new embryo itself (Spemann, '18, p. 477). But this 

 idea had to be modified in the light of the following experiment by Spemann 

 and Mangold ('24): Dorsal-lip material of unpigmented Triton cristatus was 

 transplanted to an embryo of T. taeniatus of the same age. The latter species 

 is pigmented. This experiment demonstrated that the neural plate tissue of 

 the secondary embryo was almost entirely derived from the host and not from 

 the transplanted tissue. Consequently, this experiment further suggested that 

 the organizer not only possessed the ability to organize but also to induce host 

 tissue to differentiate. Induction of neural plate cells from cells which ordi- 

 narily would not produce neural plate tissue thus became a demonstrated fact. 



The concept of an organizer in embryonic development had profound im- 

 plications and stimulated many studies relating to its nature. Particularly, 

 intensive efforts were made regarding the kinds of cells, tissues, and other 

 substances which would effect induction of secondary neural tubes. The re- 

 sults of these experiments eventually showed that various types of tissues and 

 tissue substances, some alive, some dead, from many animal species, including 

 the invertebrates, were able to induce amphibian neural plate and tube forma- 

 tion. (See Spemann, '38', Chap. X and XI; also see fig. 196A, B and compare 

 with fig. 193.) Moreover, microcautery, fuller's earth, calcium carbonate, silica, 

 etc., have on occasion induced neural tube formation. However, the mere in- 

 duction of neural tube development should not be confused with the organizing 

 action of normal, living, chordamesoderm-entoderm cells of the dorsal-lip 

 region of the beginning gastrula. The latter's activities are more comprehensive, 

 for the cells of the dorsal-lip area direct and organize the normal gastrulative 



