T8 THE A3IERICAN MONTHLY [March 



measuring only au average of 10 microns at the 20-day 

 stage. This would be expected from the physioU)gical 

 conditions of inactivity. 



Facts from Adult Forms. — As the skeletal muscle is 

 looked upon as the most highly developed, consequently 

 an examination of striped muscle which is not voluntary 

 may also throw some light upon the subject under con- 

 sideration. Of these forms, the cardiac is most sugges- 

 tive; the fibre in it is much shorter and contains only 

 one centrally placed nucleus. This then is less special- 

 ized than the striped muscle fibre of the frog for one 

 centrally placed nucleus is a characteristic of the 

 plain muscle fibre. The question arises, is there any 

 striped muscle, otherwise placed, which may show some 

 diflference? The muscle in the esophagus offers a basis 

 for comparison. Sections both longitudinal and trans- 

 verse have been examined from the various parts of the 

 tubes of some of the animals already noted. The esopha- 

 gus of the frog has only plain muscle. The rabbit's 

 esophagus has plain muscle fibre at the stomach end, but 

 this gradually changes toward the other end where the 

 striped muscle fibre is like the skeletal muscle in the 

 position of its nuclei. The average per cent of nuclei 

 imbedded in the sarcous substance is li per cent, but 

 the variation from the middle of tube to the mouth end 

 is from 4 per cent to per cent. Nothing like cardiac 

 muscle, in the gradual change, could be observed, though 

 some writers state that fibres become short toward the 

 middle of the esophagus. The cat's esophagus has also 

 plain muscle at the stomach end and gradually changes 

 to striped muscle toward the other end. It has an 

 average of 20 per cent of nuclei imbedded in the sarcous 

 substance. This large number may be due to the fact 

 that the sections, from which the observations were 

 made, were of tissue quite far down this tube. Taking 

 it, however, as correct on the whole, the position of the 



