GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



406 



In the absence of any mesoblastic component, the explant of neural tissue always forms 

 "a solid or globular mass, containing a large number of neuroblasts" (Fig. 68, b) ordered 

 in strata originating from a germinative layer surrounding a narrow ependymal lumen. 

 The picture is the same, whether the mass is naked or surrounded by epiblast. With 

 the simultaneous explantations of some notochordal or somitic material, some white 

 matter may appear, preferably near muscle cells (Fig. 68, c, d). An association of neural 

 tissue and mesenchyme only is often encountered in explants cut through the whole 

 archenteric roof and secondarily deprived of the median strip of notochordal tissue. In 

 these cases, the proliferation of neuroblasts is considerably reduced and a variable expan- 

 sion of the ependymal layer takes place (Fig. 68, e, f, g). 



A significant change occurs when the spinal cord happens to be accompanied by a 

 mixture of mesenchyme and other mesoblastic derivatives, especially myoblasts. In this 



l^ML 



Fig. 69. Effects of the neighboring tissues on the shape and differentiation of a neural tube. 

 (a, b, c) Associated influence of myoblasts and mesenchyme; (d, e, f) association with 

 notochord, segmented musculature, mesenchyme; (d) is of a rhombencephalic type; 

 (e, f) is more similar to spinal cord; (g, h) various degrees of neural proliferation resulting 

 from contact with different quantities of myoblasts; (i, k) direct contact with notochord in 

 a surrounding medium of mesenchyme and some myoblasts; the notochord alone does not 

 cause appreciable proliferation. From Takaya, 1956a. 



