1897.] MICEOSCOPICAL JOURNAL. 77 



There is a wide difference between these last fibres ex- 

 amined and the first; the results are expressed in the 

 following summary: — 



Average Average per ceut of nuclei im- 



' Diameter. bedded iu sarcous substauces. 



Dog-fish, 74 mierons 89 per cent 



Frog, 66 microns 87 per cent 



Turtle, 55 microns 77 per cent 



Pigeon, 24 microns 3 per cent 



Rabbit, 25 microns h per cent 



Cat, . 24 microns per cent 



This table shows a gradual change in the muscle fibre 

 from the more general to the specialized animals; the 

 size of the fibres not only gradually grows smaller and 

 generally more angular but they are more surrounded by 

 connective tissue. The nuclei gradually approach the 

 edge and in the highest forms are even pushing out, mak- 

 ing projections on the surface of the fibres. 



There is a large gap, however, between the cold 

 blooded and warm blooded animals, giving two distinct 

 groups, both in diameter of fibre and per cent of nuclei 

 imbedded in the sarcous substance. 



Facts from Embryolouical Forms. — There is a sug- 

 gestion that perhaps some intermediate forms could be 

 found — unless variation is dependent on physiological 

 conditions wholly — in developing muscle in embryos. 

 By a study into some developing tissues of the chick em- 

 bryos of 13, IP), 18 and 20 days, the series of changes ob- 

 served is both interesting and suggestive. The series 

 begins with irregular ill-defined cells (Fig. 8), which 

 in the next stage (16 days) shows clearly defined fibres 

 with centrally placed nuclei. The next stage, in transec- 

 tion shows in a cell, more than one nucleus generally 

 centrally placed and the last stage examined (Fig. 9) 

 shows most of the nuclei at the edge of the sarcous sub- 

 stance as in the adult pigeon. The cells, however, are 

 very much smaller than in the adult, the 20 examined 



