LATER DEVELOPMENT OF THE FROG. 651 



though degenerating gauglion, and the myotome is small. 

 This ganglion is lost in the adult^ and belongs to the missing 

 first spinal nerve. This is obvions from its position just in 

 front of the first neural arch. The number of myotomes now 

 present as far as the cloaca is only 9. 



I therefore conclude that the first two myotomes and the 

 rudimentary ganglion associated with the second, present in 

 a 9-millimetre tadpole, have now disappeared — that these 

 segments are represented by the two small cartilages h and c 

 — and that the third myotome and its ganglion are much 

 reduced in size, and are disappearing. 



In fig. 6 v/e have a figure drawn combining two sagittal 

 sections of a somewhat lai-ger tadpole, about 20 millimetres 

 long, in which the tissues of the hinder limb are beginning 

 to show differentiation. The limb is drawn in from a section 

 some distance farther on in the series, but the myotomes have 

 been counted so as to correspond. The section shows the 

 neural cord, cut a little on one side, the myotomes of that side, 

 and the neural arches and ganglia almost all the way from the 

 anterior end to the cloaca. The letter a points to the first 

 myotome still remaining at this stage, and is, as in fig. 5, the 

 third of the complete series. Anterior to it we see a blood- 

 vessel h, and just to the right of this blood-vessel, at/, there 

 are some cells which are evidently degenerated muscle-cells 

 of the second myotome. Two small nodules of cartilage, 

 c and d, are united to the parachordal bar by connective 

 tissue, which already shows signs of chondrification. These 

 are the same as the cartilages b and c in fig-. 5. The first 

 myotome present in this section — a — has a Avell-developed 

 nerve and ganglion, and as this ganglion is in the space 

 between the posterior end of the skull and the first neural 

 arch, it is the first spinal nerve, which disappears in the adult. 



Behind this we see ganglia, neural arches, and myotomes 

 in orderly series, and counting myotome a as the first, we find 

 that the ninth myotome gives off muscles to the cloaca. 



The limb which is drawn from another section of the same 

 animal is provided with three nerves, which we can see are 



