IDs 



THE AMERICAN NATURALIST [Vol. XLIV 



regions in the myotomes before and to the rear of them. 

 They thus mark out anterioposterior lines which repre- 

 sent the location of the future cylinders. The young 

 muscle fibers composing them retain their myofibril bun- 

 dles unchanged and the striation of these fibrils is exactly 

 like that of all the other myofibrils in other muscle fibers. 

 The multiplication of nuclei goes on by amitosis as in the 

 regular muscle cells. 



The next stages examined are seen in an embryo whose 

 age was between twelve and fourteen days. The word 

 4 'stages" is used because in such an embryo quite a range 

 of developmental steps can be found, owing to the fact 

 that the tail is still extending and consequently the ante- 

 rior electroplaxes are older than the posterior. This con- 

 dition is reversed in the adult, for here it can be seen 

 that the posterior electroplaxes advance further in size 

 and degree of specialization than the anterior ones did. 

 A young stage in this embryo shows that the groups of 

 cells in each myotome which are forming the electro- 

 plaxes have become so closely approximated that they 

 form a larger syncytium composed of the several smaller 

 syncytia or muscle fibers which went to form it. This 

 mass has assumed a rather distinct, elongate, spindle- 

 shaped form and each one has increased in length so as to 

 overlap its neighbor both ahead and behind it by a third 

 or more of its length. The myofibrils show a tendency to 

 occupy the central core of the young electroplax and are 

 still striated. In this stage the nerve supply can be seen 

 approaching the spindle from somewhat behind its mid- 

 dle and coining in contact with it at about the junction of 

 its middle and posterior thirds. 



The older electroplaxes in this same stage show a 

 change. This change consists in a segregation of the 

 myofibrils in the central core of the mass, where they still 

 run parallel with each other and in a straight anterior- 

 posterior direction; except in the now swollen middle 

 third of the structure where they have assumed a wave- 

 like direction. Furthermore, they have lost the larger 

 part of their striation, this feature being retained only in 



