YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 39 



The first neuromere, immediately behind the isthmus, is small and only slightly marked 

 out. It is wedge-shaped, the apex being placed ventrally. It includes in the apex a slight 

 extent of the brain floor, not having been yet forced entirely on to the side of the brain by 

 development of surrounding parts, as in Broman's case. 



The second neuromere is very large and bulging and causes a marked swelling of the 

 floor of the brain in this region, where it gives the first indication of the location of the 

 future pons. This neuromere is wider ventrally than it is above, to accommodate itself 

 to the neuromere on each side of it, which is wedge-shaped, with the broad end dorsal. 

 Associated with this second neuromere is the Gasserian ganglion, which is quite large. It 

 is connected to the neuromere, but peripherally does not give indication of the three divisions 

 of the nerve. 



The third neuromere is large, though not so large as the second. It still includes a 

 share of the brain floor, while in the stage described by Broman this neuromere has been 

 forced wholly on to the lateral wall. It agrees with Broman's case in being wedge-shaped, 

 with the narrow end directed ventrally. 



The fourth neuromere is very large and causes a bulging of the floor of the brain below 

 the level of the third. Associated with this neuromere is the acusticofacial ganglion, the 

 two parts of which are readily recognizable. The anterior portion, forming the facial 

 ganglion, lies just in front and almost in contact with the anterior end of the otocyst. It is 

 connected to the brain and peripherally extends (plate 1) right out into the second branchial 

 arch, which is rather remarkable for an embryo of this age. The acustic ganglion lies 

 directly behind and in contact with the facial and is fitted in between the otocyst and the 

 wall of the brain. It is quite small. 



The fifth neuromere is similar in shape to the fourth, being rather rhomboidal. No 

 structures are connected with it, but the posterior half of the otocyst lies opposite to it, 

 the anterior half being opposite the fourth neuromere. 



The roots of the seventh and eighth cranial nerves are invariably found opposite and 

 attached to the fourth neuromere, as described here, but the position of the otocyst seems 

 to vary, being usually opposite the fifth neuromere, as seen in the embryos described by 

 Broman (1896) and by Mrs. Gage (1905). In this case, however, approximately half 

 of the otocyst is anterior to the fifth neuromere, being opposite the fourth, and in intimate 

 contact with both the auditory and facial portions of the acusticofacial ganglion. In 

 Keibel and Mall's Embryology (1912) the location of the auditory vesicle is placed opposite 

 the fifth neuromere only. Of course the embryos I have referred to are all considerably 

 older than the ones here described, so that there is plenty of opportunity afforded for a 

 change in the relations from those found at this time. 



The sixth neuromere and those back of it resemble very much the neuromeres of the 

 spinal cord, and this posterior part of the medulla is indistinguishable from the cord except 

 for one feature, which is the commencing formation of the fourth ventricle, as shown by 

 the expanded dorsal portion of the lumen. Each neuromere from here back is similar and 

 includes a section of the whole nervous tube, but is not marked on the roof, as this is not 

 yet separated from the ectoderm. The sixth neuromere has connected with it on each 

 side the ganglion of the glossopharyngeal nerve. This ganglion is fairly large and extends 

 about half the distance toward the branchial arch with which it is to be associated. 



The seventh neuromere has connected with it the ganglion of the vagus. This is not 

 nearly as large or as distinct as the ganglion of the glossopharyngeal nerve, but is quite 

 recognizable. The sixth and seventh neuromeres have been constantly associated with the 



