A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



In 1892, Minot 36 reviewed the earlier literature on this subject and made the 

 general statement that "the entire medullary tube undergoes a segmentation by 

 a series of alternating slight enlargements and constrictions." He adds: 



"They appear first in the hind-brain and cervical region, and from there they appear 

 progressively toward the fore-brain and the tail .... The medullary tube becomes 

 slightly constricted between each pair of segments and slightly enlarged opposite each 

 intersegmental space. Each intersegmental dilation is a neuromere .... Each neuro- 

 mere produces a pair of nerves, but when the first trace of roots appears, they are seen 

 to spring from the constriction between the neuromeres, but later from the neuromere." 



Minot believes, therefore, that the so-called neuromeres of early stages represent, 

 not true neuromeres, but the caudal and cephalic halves of two adjacent neuromeres. 



An internal view of the brain is shown in plate 7, figure 5. The cavity of the 

 prosencephalon is large and deep. Towards its anterior end the cavity of the 

 optic vesicle is seen extending outward and backward. Posteriority this is marked 

 off from the forebrain cavity by a sharp indented ridge. The slight eminence of 

 the ventral wall which will probably give rise to the infundibulum is again seen, 

 being placed at the anterior end of the notochord. The walls of the rhomben- 

 cephalon, when viewed from within, show the negative impression of the neuro- 

 meres, the "rhombic grooves" of Streeter 42 . 



The question of neuromeres in mammalian embryos is still an open one. 

 Those of the spinal cord are undoubtedly related to the metameres, i. e., represent- 

 ing true or parts of true morphological units of the medullary tube. Similar evi- 

 dence concerning the "neuromeres" of the rhombencephalon is lacking. Although 

 the arrangement of cerebral nerves is not contradictory to this view, i. e., each 

 neuromere, with the exception of the first, receiving afferent fibers and sending out 

 efferent fibers (Johnson) 21 , the muscles which the efferent nerves supply, the source 

 of these muscles, and the "neuromeres," from which the efferent nerves spring, 

 have not yet been demonstrated to coincide. In fact, the evidence, so far as 

 gathered, contradicts this arrangement. Streeter 42 believes that the rhombic folds 

 are not related to the metameric system, but is inclined to the view that " they may 

 be fitted in with and form a part of the branchiomeric system." 



He adds: 



"The one discordant feature is groove d (5th neuromere), which has no correspond- 

 ing branchial arch." 



Neal 38 , after extended observations and studies of head segmentation, in 

 embryos of lower vertebrates, concludes that neuromeres offer no criterion for the 

 determination of segmentation of the vertebrate head. 



CEREBRAL NERVES. 



As in the Janosik and Thompson embryos, the ganglia of the trigeminal and 

 acustico-facial nerves are clearly present. Of the two, the trigeminal arises a 

 little higher dorsally than the acustico-facial. Each is made up of a cluster of 

 nuclei which are more compact and deeply staining than the mesenchymal cells 



