1172 EMBR YOLOG F. 



Before leaving the subject of the visceral arches and clefts it is necessary to 

 mention that the clefts disappear early in embryonic life, with the exception of 

 portions of the first, which remain permanent the inner portion, or the Eustachian 

 tube and tympanum ; the outer, as the external auditory meatus, while the septum 



Malleus. 



Tympanic ring. 

 Mandible. 



Meckel's cartilage. 



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Hyoid bone. 



FIG. 719. Head and neck of a human embryo eighteen weeks old, with Meckel's cartilage and hyoid bar 

 exposed. (After Kolliker.) 



between the two portions becomes invaded by mesoblast and forms the membrana 

 tympani. 



Development of the Nervous Centres and the Nerves. The medullary or neural 

 groove already described (page 1155) is the rudiment of the cerebro-spinal axis. 

 As has been seen, this groove is converted into a canal (the neural canal) : its 

 cephalic end becomes dilated into a sac, from which the brain is developed ; the 

 remainder forms the spinal cord. The cavity of the canal becomes the central 

 canal of the spinal cord, and that of the upper dilated portion the ventricles of the 

 brain. The wall of the canal, formed of epiblastic cells, undergoes great changes, 

 and from it the nervous matter and neuroglia are developed. It consists at first 

 of a layer of columnar epithelium, covered on its exterior by a basement-mem- 

 brane. The wall becomes thickened, partly by the elongation of the columnar 

 cells and partly by the formation of new cells. The elongation of the columnar 

 cells, now called spongioblasts, is followed by the breaking up of their outer ends 

 into a reticulum, which is termed the myelo-spongium, and eventually forms the 

 neuroglia. The new cells which are formed appear between the inner ends of 

 the columnar cells as rounded masses, which speedily divide, and are termed neuro- 

 blasts ; they become pear-shaped, and projecting from each of them is a tapering 

 process which perforates the basement-membrane. These neuroblasts are the primi- 

 tive nerve-cells, and their tapering processes the rudimentary axis-cylinders of the 

 cells (Figs. 721 and 722). 



It will be convenient, in the first place, to trace the changes which take place 

 in the cavity of the cerebro-spinal axis, ignoring for a time those which go on 

 in the enclosing wall. But before doing so, it is necessary to mention that, in 

 consequence of the curve which the cephalic portion of the embryo undergoes, a 

 marked bend forward of the canal takes place, so that the plane of the ventricles 

 is almost at right angles with the long axis of the central canal of the cord. 



The early stage thus consists of a hollow sac, which is the rudimentary brain, 

 and a hollow canal, which is the rudimentary cord ; the sac and the canal freely 

 communicate with each other. The sac first of all becomes elongated ; then two 

 constrictions appear in it, which partially divide it into three ; these are named 

 anterior, middle, and posterior cerebral vesicles, or the fore-brain, mid-brain, and 



