132 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



Lying next to the second neuromere is the ganglion of the trigeminal nerve; next 

 to the fourth neuromere is the ganglion of the acustico-facial. The auditory 

 vesicle lies opposite the fifth neuromere and partly overlaps the sixth; the latter 

 is in process of giving off the cells of the glosso-pharyngeal ganglion. Beginning 

 with the seventh, the remaining neuromeres are longer, being equal in length to 

 the body segments. They do not lie within the segments themselves, however, 

 but are arranged intersegmentally, the crest of each neuromere being placed 

 opposite an intersegmental cleft. This arrangement begins with the first body 

 segment and continues throughout the embryo. Above the first segment are 

 85 neuromeres. 



The number of neuromeres belonging to the rhombencephalon can not be ascer- 

 tained from this specimen alone. To determine this point, the author undertook 

 a separate study of neuromeres based upon young human, pig, sheep, and cat 

 embryos. Although this study is yet incomplete, it is evident that the last rhombic 

 neuromeres stand out more clearly in certain specimens than in others; in some 

 their presence is extremely doubtful. Just what this may be attributed to I am 

 not able to state; it may be due to the stage of the specimen examined, or their 

 presence on the one hand or absence on the other may be regarded as artificial. In 

 those specimens which show distinctly a complete series of neuromeres, I have 

 found that the first cervical ganglion is constantly related to the tenth neuromere. 

 It appears evident, therefore, that the first 9 neuromeres belong to the rhomben- 

 cephalon. It is to be noted that the first pair of somites in the embryo under dis- 

 cussion begins approximately opposite the crest of the eighth neuromere. 



Of the previous description of neuromeres in young human embryos, those 

 which appear to accord most closely with my own are the ones of Gage 12 and 

 Watt 48 . In a description of an embryo of 28-29 pairs of somites, Mrs. Gage finds 9 

 folds or neuromeres in the rhombencephalon; of these the second is associated 

 with the trigeminal nerve; the fourth with the auditory and facial nerves; the 

 fifth is opposite the auditory vesicle; the sixth is in relation to the glosso-pharyn- 

 geal nerve; the seventh to the vagus; and the eighth and ninth to the accessory 

 nerve. In regard to the spinal cord she states: 



"Beyond the clearly formed folds, above discussed, there occur several others, each 

 corresponding with an enlarged part of the ganglionic cord. As this cord has no further 

 indication of dorsal nerve roots, the exact relations can not be determined. Moreover, 

 the following total folds in the myel (spinal cord) are not strongly marked, and in other 

 specimens it is only in favorable sections that they can be seen at all. 1 ' (pp. 435-436.) 



'Watt 48 , in describing twin human embryos of 17-19 pairs of somites, simi- 

 larly shows 9 neuromeres in the rhombencephalon. The tenth neuromere, In- 

 states, is opposite the first cervical ganglion. The results I have obtained with 

 other mammalian embryos confirm this observation. Watt also shows spinal 

 neuromeres extending along the medullary tube as far as the eleventh spinal 

 ganglion. 



