88 



THE CELL AND PROTOPLASM 



in the central nervous system are hetero- 

 geneous in composition. Underlying the 

 fore brain is the pharyngeal roof and some 

 mesenchyme, and under the rest of the 

 brain and spinal cord the notochord oc- 

 cupies the midline with the myotomes on 

 each side. All of these factors are known 

 to have a specific effect on the overlying 

 neural plate (Lehmann 1926, 1928, 1938). 



Since the word "organizer" connotes a 

 master regulator which creates the organi- 

 zation, and since there are in the course of 

 development many actions of the same 

 general character that could hardly be ac- 

 corded such a role, it is perhaps more ap- 

 l^ropriate to use the word "induction" to 

 denote processes of this kind. Outside the 

 area of the central nervous system there are 

 many other ectodermal organs — balancer, 

 gills, hypophysis, etc. — that are dependent 

 upon other structures for their differenti- 

 ation. Whether the ectodermal structures 

 just referred to arise in situ through some 

 localizing factor in the ectoderm itself, or 

 whether they are produced by reaction 

 with the underlying tissue is still uncertain 

 in some cases. 



An experimental study of the develop- 

 ment of the ear shows that, beginning with 

 the late gastrula or early neurula stage, at 

 least four factors are concerned in its 

 differentiation (Harrison 1935a, 1938a). 

 Three of these lie within its internal en- 

 vironment. The cellular material is, as is 

 well known, supplied by the ectoderm. 

 While an ear vesicle may arise from any 

 ectoderm of the gastrula, in the early 

 neurula stage the cells of the auditory 

 placode, out of which it normally develops, 

 are already predisposed toward ear forma- 

 tion (otogenesis). The underlying meso- 

 derm is still involved at this time and it is 

 probably this layer, acting at a still earlier 

 stage, that has induced the just mentioned 

 condition in the ear placode. A second ef- 

 fective factor in the internal environment 

 of the developing ear is a certain spot in 

 the hind brain region of the neural tube 

 with which the ear placode is brought into 

 contact as the neural folds rise and close 

 over. A fourth factor is the position in the 



embryo in which the ear vesicle develops. 

 By means of grafting experiments all of 

 these factors have been varied, but the re- 

 sults do not indicate that they singly have 

 any markedly specific role to play, except 

 that the hind brain seems to be significantly 

 associated with the development of the en- 

 dolymphatic duct and sac. The develop- 

 ment of a normal ear vesicle seems rather 

 to be dependent on a series of factors act- 

 ing in turn, and furthermore the orienta- 

 tion within the organism of at least two of 

 them — the ear placode and the myelen- 

 cephalon — have at the appropriate time an 

 effect upon the asymmetry of the ear that 

 develops. 



Again nothing is known with regard to 

 the actual processes involved in the de- 

 velopment of this organ and the nature of 

 the action of one part upon the other. 

 There is evidence, however, that an orienta- 

 tion of some structural element of the ear- 

 plate cells takes place about the time of 

 closure of the neural folds (Harrison 1936). 



There is a certain similarity between the 

 formation of the neural groove and that of 

 the auditory pit in that cells elongate in 

 a direction perpendicular to the surface 

 and a vesicle is ultimately folded off from 

 a plate. There are differences in degree of 

 elongation, in shape, and in the fact that 

 the neural folds rise above the surface, 

 while the auditory pit sinks in and the 

 folds remain even with the surface. One 

 might again assume a hydration in the pro- 

 tein lattice of the auditory cells and a con- 

 sequent change in the lattice spacing, with 

 a resulting change in the shape of the cells 

 which brings about a modification of the 

 shape of the rudiment from a plate to a 

 vesicle. Subsequent changes, which pro- 

 duce the internal ear or labyrinth out of a 

 simple vesicle involve complicated folding 

 and constriction of its walls. Certain con- 

 ditions in the internal surroundings, pos- 

 sibly the failure of the endolymphatic duct 

 and sac to develop, often bring about os- 

 motic disturbances in the vesicle, which 

 then swells considerably and becomes 

 cystic. Normal folding fails to take place 

 and the sensory areas develop incompletely. 



