GENEKAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 79 



narrow orifice closes. Out of the auditory pit there has arisen a 

 closed auditory sac (6), which then detaches itself completely from its 

 parent tissue, the epithelium of the surface of the body. Afterwards, 

 simply hy means of the unequal growth of its different regions, by 

 means of constrictions and various evaginations, it acquires such an 

 extraordinarily complicated form, that it has justly received the 

 name of membranous labyrinth, as will be shown in detail in another 

 chapter. 



The development of the central nervous system may serve as 

 the last example of invagination. Spinal cord and brain take their 

 origin at an early epoch from the layer of epithehal cells which limits 

 the outer surface of the body of the embryo. A narrow band of this 

 epithelium lying along the axis of the back becomes thickened, and is 

 distinguished from the thinner part of the epithelium, which produces 

 the epidermis, as the medullary plate (fig. 41 A mp). Inasmuch as 

 the plate grows more rapidly than its surroundings, it becomes in- 

 folded into a gutter which is at first shallow, the medullary groove. 

 This becomes deeper as a result of further increase of substance. At 

 the same time the edges (fig. 41 B inf), which form the transition 

 from the curved medullary plate to the thinner part of the cellular 

 membrane, become slightly elevated above the surrounding parts, and 

 constitute the so-called medullary folds. Subsequently these grow 

 toward each other, and become, so apposed that the furrow becomes 

 a tube, which still remains temporarily open to the outside by means 

 of a narrow longitudinal fissure. Finally, this fissure also disappears 

 (fig. 41 C) ; the edges of the folds grow together ; the closed medullary 

 tube (w), like the auditory vesicle, then detaches itself completely 

 along the line of fusion (suture) of the cell-membranes of which it 

 was originally a component part and becomes an entirely independent 

 organ (w). 



Let us now examine somewhat more closely the mechanism of the 

 fusion and detachment of the neural tube. 



The two medullary folds are each composed of two layers, which 

 are continuous with each other at the edge of the fold, — the thicker 

 medullary plate {mp), which lines the furrow or tube, and the thin- 

 ner epidermis (ep), which has either a more lateral or a more super- 

 ficial position. When, now, the folds come into contact, they fuse, 

 not only along a narrow edge, but over so extensive a tract that 

 epidermis is joined to epidermis, and that the edges of the medullary 

 plate are joined to each other. The medullary tube thus formed, 

 and the continuous sheet of epidermis that stretches across it, are by 



