No. 4-] THE OCCIPITAL REGION OF THE TROUT. 153 



appeared. It undoubtedly corresponds to the temporary myo- 

 tome pair found by Harrison in Salmo. 



The anlage of the axial skeleton consists at this time of con- 

 densed mesoderm aggregated on either side of the chorda, 

 especially along the dorso-lateral and ventro-lateral lines, extend- 

 ing up the neural canal nearly to the top of the spinal ganglia 

 and broadening anteriorly into the parachordals. This anterior 

 broadening begins opposite the fifth myotome. The mesoderm 

 shows no trace of segmentation, except that it is marked at 

 intervals by ridge-like vertical thickenings corresponding to the 

 myosepta. Such ridges are present also on the parachordals 

 opposite the myosepta between segments 2 and 3, 3 and 4, 4 

 and 5. Cartilage is present in an embryo nine days older; it 

 has the form of paired rods (neural arches) flanking the spinal 

 cord. Each rod lies with its ventral end in a myoseptum, but 

 crosses the myotome obliquely, so that its dorsal end lies in or 

 near the next anterior myoseptum. The foremost rods cross 

 the fifth myotome pair ; they are considerably smaller than 

 the others, and are closely connected by condensed mesoderm 

 with the parachordals. They are obviously the anlage of the 

 neural arch (occipitalbogen), which in the Salmonidae fuses 

 with the skull. In specimens of twenty-one days after hatch- 

 ing, no fusion has yet taken place. The parachordals in my 

 youngest embryos are largely chondrified ; the cartilage exists 

 as a continuous mass which shows no suggestion of segmenta- 

 tion. According to Stohr, that portion which lies behind the 

 otic capsules ("hintere Parachordalplatten ") chondrifies on 

 either side from a single center. 



We come now to the nerves. The two temporary segments 

 are altogether without them, but the first permanent segment 

 has a rudimentary one ; the second permanent segment has a 

 typical spinal nerve which differs in no respect from the suc- 

 ceeding ones, and in my oldest embryos (twenty-one days after 

 hatching) shows no sign of degeneration. Harrison states that 

 in salmon somewhat older the dorsal root is atrophied. The 

 rudimentary nerve of the first permanent segment is present in 

 my youngest embryos. It is better developed on the right side, 

 and here has the typical structure of a spinal nerve, differing 



