152 WILLCOX. [Vol. II. 



I take as the basis of my description the youngest of the 

 trout embryos. The following data will indicate their approxi- 

 mate age : The length was a little less than i cm. ; the spinal 

 ganglia were formed, but the anterior ones were still connected 

 by a longitudinal commissure with the vagus ; the operculum 

 had covered the first branchial arch. The characteristic struc- 

 tures of a segment at this stage are : {a) a pair of myotomes ; 

 {b) a spinal nerve with a ventral root and a dorsal ganglion 

 which is not as yet directly connected with the spinal cord by 

 a dorsal root ; {c) a portion of condensed mesoblast forming the 

 anlage of the axial skeleton. These structures I will take up 

 successively in order to compare the condition in the most 

 anterior segments with that which obtains farther back. 



The myotomes are well differentiated and in general similar ; 

 they extend forward to the foramen for the vagus, the first one 

 lying close behind the condensed mesoderm which invests the 

 ear. Striation of the fibers is already present but is not uni- 

 form, being generally more pronounced in the deeper parts. 

 The time at which striation appears seems to be variable ; in a 

 second specimen nine days older than this it is barely indicated. 



The anterior three myotomes lie laterad of the parachor- 

 dals and accordingly represent post-otic cephalic segments; they 

 resemble the posterior ones but lie more laterad, as if pushed 

 out by the enlargement which forms the hind brain. In front 

 of the lateral portion of the most anterior one on the right side 

 is a small triangular mass of tissue in which a few unstriated 

 longitudinally disposed muscular fibers are to be seen, and 

 which undoubtedly represents another nearly atrophied myo- 

 tome, making the niimber of post-otic cephalic segments four. I 

 propose at the earliest opportunity to examine younger embryos 

 in the hope of finding this first myotome better developed. In 

 salmon embryos I have found no trace of it, though I have 

 not made a search exhaustive enough to warrant me in assert- 

 ing that it is absent. In trout embryos one day older than the 

 one just described, it has disappeared ; the succeeding (second) 

 myotome pair also eventually atrophies, though I am unable 

 to say just when. Seven days after hatching it shows no trace 

 of degeneration ; in trout of fourteen days it has entirely dis- 



