Charles Eussell Bardeen and Warren Harmon Lewis 11 



and II, the occipital myotomes have been determined by a careful study 

 of serial sections. 



The cervical myotomes are those developed in conjimction with the 

 cervical spinal nerves. In the vast majority of instances these are eight 

 in number. The occasional absence in the adult of a cervical vertebra 

 indicates that in the embryo less than the normal number of cervical 

 spinal segments sometimes develop. In all of the embryos we have 

 studied, the arm-bud begins its development opposite the fifth to the 

 eighth cervical segments and opposite the following spinal segment. 

 The most caudal myotome lying opposite the arm-bud may, therefore, 

 be taken to represent the first thoracic segment. 



Between the base of the arm-bud and that of the leg-bud, as a rule, 

 eleven myotomes intervene. A marked exception to this is found in 

 Embryo CXLVIII, where but nine myotomes seem to lie in this region. 

 In other instances, twelve myotomes have been pictured. Such is 

 apparently the case in the His embryo B (Atlas, Taf. I, Fig. 1) and in 

 the His embryo Pr (Atlas, Taf. XIII, Fig. 4). It is difficult to deter- 

 mine this with certainty from the figures. Variation in the number of 

 myotomes intervening between, the regions of the arm and leg corre- 

 sponds with well-known variations in the length of the spinal axis in 

 the adult.' , 



* Bardeen, Costo-vertebral Variation in Man, Anat. Anz., 1900. 



In the His wax-models of young human embryos, Embryo Lr (No. 6) shows nine 

 myotomes between the regions of the swellings which indicate the arm- and leg-buds. 

 Embryo A shows fifteen myotomes between the arm and leg areas. The number of 

 the myotomes in the thoraco-abdominal region in each of the models is probably in- 

 correct. Fig. .5, Plate XX, of the His Atlas seems to show the usual number of 

 thoraco-abdominal myotomes in Lr. Fig. 2, Plate I*, shows twelve instead of fifteen 

 thoraco-abdominal myotomes in A. The model of A presents, in case of the thoraco- 

 abdominal myotomes, conditions characteristic of the pig. Out of twelve young pig 

 embryos of various sizes, in eleven instances we have found sixteen myotomes inter- 

 vening on each side between the arm and leg regions, and in one instance fifteen. In 

 the pig, therefore, five more body segments than in man lie between the arm and leg 

 areas. This is also to be seen in the adult. In the pig there are two more thoracic 

 segments than in man, and, as indicated by the nerve distribution to the abdominal 

 wall, three more abdominal segments. The third lumbar nerve in the pig corres- 

 ponds in distribution somewhat to the first lumbar in man, but it has a more exten- 

 sive abdominal distribution. While therefore in man the first lumbar segment is 

 counted as belonging to the leg area, in the pig the third lumbar segment may be 

 considered to belong to the thoraco-abdominal region. There are six lumbar verte- 

 brie in the pig. The furcal nerves are the fifth and sixth lumbar usually, but some- 

 times the fifth, probably sometimes also the sixth, may be the sole furcal nerve. 



In the Keibel " Normentafeln " of the pig the number of myotomes pictured be- 

 tween the arm and leg areas varies from fifteen to eighteen in different embryos. It 



