40 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



ninth and tenth cranial nerves, respectively, so that these findings are in full accord with 

 those of other investigators. 



The next two neuromeres are included by Mrs. Gage as folds 8 and 9 of the medulla 

 oblongata. She finds them as dorsal pockets only of the neural tube, but in Embryo VI 

 they include the whole tube. These two folds lie in the region of the first three pairs of 

 somites, the neuromeres being opposite the intersegmental clefts. The origin of the 

 accessory portion of the eleventh nerve is stated to arise from these neuromeres, but no 

 evidence of this nerve is here present. The ganglion crest of the neuromeres is continuous 

 with the ganglion of the vagus in front. There is no trace of the hypoglossal nerve, whose 

 fibers must come from these neuromeres, as they lie opposite the segments later included 

 in the occipital region of the skull. 



Accepting the number of neuromeres in the rhombencephalon as 9, there are to be 

 found 11 (plate 1) in the spinal cord, the last one being opposite the thirteenth interseg- 

 mental cleft. There is no line of demarcation between brain and cord. The condition of 

 the cord changes slowly and very gradually, becoming less in its various diameters as it is 

 followed caudad. Back of the last neuromere it gradually becomes circular in section. In 

 front of this it is much elongated dorsoventrally. At the level of the last somite the cord 

 becomes open dorsally, forming the posterior neuropore. The walls are at first close 

 together and the gutter formed is narrow and deep, but this gutter gradually becomes 

 shallower and wider and finally, in the tail region, all trace of the lips of the neural groove 

 are lost and the nervous system terminates as a flat plate which rapidly blends with the 

 tissues of the primitive streak. Shortly before this fusion, from the under surface of the 

 medullary groove to the notochord, the remains of the neurenteric canal pass, evident as a 

 solid cord of cells, already described (plate 1 and plate 2, fig. 10). 



The whole extent of the nervous system on its dorsal surface is still fused to the ecto- 

 derm; nowhere is there as yet any separation. In all the closed portion of the tube in the 

 region of the spinal cord the neural crest is evident and from it there passes a spinal ganglion 

 (plate 1) between every two somites as far back as the last neuromere. Back of this point 

 no ganglia are yet evident. There is a neural crest also in the posterior part of the hind- 

 brain, from which the ganglia of the glossopharyngeal and vagus nerves are formed. This 

 crest seems to be directly continuous with that of the spinal cord and would thus agree with 

 the views of Dohrn (1901), who found them continuous. 



No motor nerves are yet in evidence in any portion of the nervous system. 

 Histologically the whole closed portion of the nervous system appears similar. There 

 is an internal limiting membrane lying next the lumen, then the layer of ependymal cells. 

 External to this is the mantle zone, in which the cells are apparently in the indifferent stage. 

 Outside of this again is the marginal zone, formed of protoplasmic processes of the cells and 

 containing absolutely no nuclei. This zone is not developed in the dorsal and ventral 

 portions of the tube, but is well marked in the lateral regions. There is no division yet 

 into the dorsal (or alar) and the ventral zones, corresponding to the sensory and motor 

 regions of the cord. Inclosing the whole nervous system throughout is a very definite, 

 thin, structureless, external limiting membrane. The nuclear layers in the walls of the 

 neural tube vary from 4 to 6 in number. 



The nervous system of Embryo V is so similar in every way to that of its twin. Embryo 

 VI, that no separate description of any part is necessary, and it can be dismissed with only 

 a few remarks regarding some slight individual differences. The appearance on section of 

 the various parts of the brain and cord is almost identical in the two embryos. Embryo V, 



