BRAL DISCUSSION OK TllK PI >F DEVELOPMENT 7'. 



narrow orifice closes. Out of tin- audit. u-y pit th- n- has arisen a 



ed auditory sac (b), which then d.-taches itself complete! 

 part-lit tissue, the epithelium <>f the surface of the body. A: 



simply by means of tin- unequal growth of its dillerent regions, by 

 means of coiiMrictions and various evaginat ions, it acquires such an 

 <-.\traorilin:irily complicated form, that it has justly n-ceivtd the 

 name of membranous labyrinth, as will In- >ho\vn in detail in another 

 chapter. 



The development of the central nervous system n e at 



the la>t example of in va^ination. Spinal cord and brain take their 

 origin at an early epu h from the layer of epithelial cells which limits 

 tin 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 

 iiruished from the thinner part of the epithelium, which produce- 

 tin- epidermis, as the medullary plate (fig. 41 A ///]>). Inasmuch a> 

 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 fun her increase of substance. At 

 the same time the edges (fig. 41 B ////), 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 

 conMituto 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 d i.-appears 

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

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

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

 vraa originally a component part and becomes an entirely independent 

 organ (n). 



Let us now examine somewhat more closely the mechanism of the 

 fusion and detachment of the neural tube. 



The two medullary folds are each coin] 9, 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 sin 

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

 not only along a narrow edge, but : M\- a tract that 



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

 plate are joined to each other. The medullary tul>e thus formed, 

 and the continuous sheet of epidermis that stretch* > acn u it, arc by 



