462 PATTERNS AND PROBLEMS OF DEVELOPMENT 



IS INDUCTION NECESSARY FOR NEURAL-PLATE FORMATION? 



From his experiments Goerttler (1925, 1926, 1927) concluded that 

 orientation of an inductor implant may be favorable or unfavorable, ac- 

 cording to its relation to the regional cell migrations; consequently, he 

 regarded these migrations as factors in determination of the neural plate 

 and the presence of underlying chorda-mesoderm as not absolutely neces- 

 sary. Holtfreter (19330, b) found orientation without effect and con- 

 cluded that induction, not the migrations, is the essential factor in de- 

 velopment of the neural plate. However, Goerttler has described and fig- 

 ured cases in which more or less development of neural plate and neural 

 tube occurred when the whole dorsal lip region had been removed before 

 its invagination. According to Lehmann (1926, 1928, 1929), there is a 

 quantitative relation between degree of defect in chorda-mesoderm, re- 

 sulting from experimental removal of part of the dorsal lip region before 

 invagination, and degree of development of the overlying region of the 

 neural plate; but this relation differs at different levels. Anteriorly only 

 the most extreme defects in the inductor determine defects in neural 

 plate, but farther posterior the neural plate is less independent. Lehmann 

 concludes that there is labile determination of the neural plate at the 

 beginning of gastrulation, decreasing from the anterior region posteriorly, 

 and therefore independent of underlying chorda-mesoderm. 



Isolation of parts of the embryo before, or at beginning of, gastrulation 

 give different results. Apical regions isolated in water never develop neu- 

 ral plate, nor do two such regions unite (Spemann, 1936, p. iii). Cul- 

 tured in balanced salt solutions, entoderm and mesoderm show a high 

 degree of independent differentiation (W. Erdmann, 1931; Holtfreter, 

 igsia, b), and Erdmann found that presumptive neural plate similarly 

 cultured develops into neural tissue, both in urodeles and anura; but 

 Holtfreter maintained that it always forms epidermis, with at best only 

 minute traces of neural tissue. However, he did obtain neural and other 

 tissues from a group of four cells from the lower part of the apical hemi- 

 sphere, isolated by destroying the other cells and developing in salt solu- 

 tion, surrounded by the debris of the killed cells. Erdmann and Holt- 

 freter agree that presumptive neural plate cultured in explant does not 

 give rise to chorda-mesoderm. Material of the dorsal lip shows a con- 

 siderable capacity for reconstitution; an explanted half may give rise to 

 a bilaterally symmetrical complex, consisting of a chordal strand with 

 muscle and, in the anterior region, neural tissue and epidermis, that is, 

 ectodermal derivatives from presumptive mesoderm (Holtfreter, 1933c). 



