226 Caroline McGill, 



bundles of fine fibrillae, so condensed in the center as to appear 

 homogeneous and becoming looser around the periphery. Other evidence 

 that coarse myofibrillae may split up into fine is the fact that in the 

 late foetus and in the adult, there is usually a decrease in the number 

 of coarse myofibrillae and at the same time an increase in fine myo- 

 fibrillae. Where, in exceptional cases, there are many coarse myo- 

 fibrillae, as shown in Figs. 17 and 29, there are usually relatively 

 fewer fine myofibrillae present. It is also possible that the fine myo- 

 fibrillae may themselves divide longitudinally. Where there are few 

 or no coarse myofibrillae present, and at the same time a rapid in- 

 crease in the number of fine myofibrillae, this would seem a reasonable 

 explanation. 



In their staining reactions, the myofibrillae vary so much during 

 development, that it has seemed better to take up this subject sepa- 

 rately. In sections stained with iron-haematoxylin and eosin, the pro- 

 toplasmic granules stain red with the eosin and retain none of the 

 haematoxylin , unless the extraction in the iron-alum has been very 

 slight. As soon, however, as the granules begin to form the more 

 condensed spindles and varicose fibrillae (as in Figs. 3, 4 and 5, gs, vf) 

 they begin in the denser portions to retain the black haematoxylin 

 stain, the looser structures still staining with the eosin. This change 

 in the staining reaction may be due to a physical rather than to a 

 chemical transformation, however. Fig. 7 shows the dense central 

 portions of the coarse myofibrillae stained intensely black by the 

 haematoxylin, the more loosely arranged ends, as well as the finer 

 fibrillae, red with the eosin. Similarly in the later stages the bundles 

 of myofibrillae often show a homogeneous center stained black (coarse 

 myofibrillae) surrounded by the fine myofibrillae staining red with 

 the eosin. 



At any stage in their development both fine and coarse myo- 

 fibrillae will stain with the iron-haematoxylin when the extraction 

 with iron-alum is not allowed to proceed too far. With the fine 

 fibrillae the tendency to retain the haematoxylin is much more pro- 

 nounced in the late embryo and in the adult than during the younger 

 stages (Figs. 22 and 23). 



