1897J MICROSCOPICAL JOURNAL. 79 



nuclei in the striped muscle fibres of the cat's esophagus 

 may be considered less specialized than that of the rab- 

 bit's esophagus. It may be regarded as an interesting- 

 fact that in the rabbit, the nuclei are quite centrally 

 situated when not at the edge where as in the cat, those 

 in the sarcous substance appear only just off the edge 

 of the sarcolemna. 



The embryonic chick esophagus affords interesting 

 gradations too. The earlier stages show distinct cells, 

 each with a central nucleus. They appear in transection 

 as very like plain muscle fibres. In an older embryo, the 

 fibres are more angular, the striated condition distinct 

 and the nuclei both centrally and marginally situated 

 as in the pigeon, with the greater per cent at the margin. 



Thus we find a series of adult structures in various 

 animals showing certain marked difi'erences. An em- 

 bryological series may be made showing variation of a 

 developing structure in one animal that corresponds in 

 general to the series of adult forms. Also intermediate 

 forms may be found in adult animals by considering some 

 part (esophagus) not so strongly voluntary in action. 



The table given above, shows that the muscle fibres be- 

 come more specialized, the higher we go in the animal 

 kingdom. In position of nuclei, the large gap between 

 the cold blooded and warm blodied animals is bridged 

 over by the developing tissues of the chick embryo. It 

 is known that striped muscle develops from cells similar 

 to plain muscle fibres. The facts given above in regard to 

 the striped muscle of the esophagus and chick embryonic 

 tissues illustrate how specialized skeletal muscles devel- 

 ope, from plain muscles and that ancestral forms may be 

 found in the skeletal muscles of the less specialized 

 animals. 



[The above work was done at the Wesley College laboratory under the 

 kindly direction oi- Miss E. J. Claypole, to whom the writer gratefully ac- 

 knowledges indebtedness.] 



