288 George L. Streeter 



The rhombic grooves present in the floor of the fourth ventricle were 

 described in the paper previoiisly mentioned (Streeter, '08), and their 

 relation to the cranial nerves was also referred to. It will only be 

 necessary here to refer to the accompanying Fig. 1., in which is shown 

 a composite sagittal view of the embryo. The six rhombic grooves are 

 indicated by the letters a. to f. Extending from the rhombic grooves 

 to the branchial arches may be seen the trigeminal, facial, glossopharyn- 

 geal and vagus nerves. The correspondence of the arches, the nerves 

 and the grooves strongly suggests the branchiomeric character of the 

 whole group. Arising from groove d. is the n. abducens extending 

 forward to the region median to the trigeminal. This is included in 

 the drawing, though it is recognized that it may have no determinative 

 connection with the fourth groove with which it stands in relation. 



The outlines between the mesencephalon, thalamencephalon and pros- 

 encephalon are easily made out, and correspond closely with the His 

 KO embryo (Tab. II, Fig. 3). It is to be noted, however, that in 

 this case the olfactory bulb is not as well developed as indicated by the 

 His drawing, the beginning olfactory pouch being represented by only a 

 slight depression in the brain wall just lateral to the lamina terminals. 



The general structure of the neural tube at tliis stage is shown in 

 a series of cross sections given in Figs. 20-26 in the monograph of His, 

 '88, mentioned above. They show the division of the wall into three 

 layers, the so-called ependymal, mantle and marginal zones; the first 

 is made up of deeply staining primitive cylindrical cells, the second 

 of clumps of budding neuroblasts, and the third is almost wholly fibrous. 

 In the mantle zone are developed the nuclei of origin of the motor nerve 

 roots, which can be outlined as shown in the reconstructions. The mar- 

 ginal zone consists of a reticulated framework through which the various 

 fibre tracts make their way. At this time the only tracts that stand out 

 prominently are the dorsal funiculi of the spinal cord, consisting of 

 processes from the dorsal roots of the spinal nerves, and in the brain 

 the corresponding tractus solitarius and spinal tract of the trigeminal 

 nerve. In addition to these a portion of the marginal zone can be out- 

 lined which contains developing longitudinal fibres, and represents the 

 so-called ventral ground bundle. In the cord it serves to connect the 

 different levels of the mantle layer. It is continued through the hind- 

 brain, and longitudinal fibres grow through it in that region, eventually 

 resulting in the three main longitudinal tracts : the fasciculus longi- 

 tudinalis medialis, the lemniscus and the tractus pyramidalis. This 



