508 Caroline McGill. 



The interstitial connective tissue arises in situ. Some of the 

 mesenchyme cells in tlie area of muscle formation do not elongate but 

 persist as the connective tissue cells. These are connected, at least 

 until a very late stage, and at times in the adult with the protoplasmic 

 syncytium of the muscle tissue, Fig. 14. In the common syncytium 

 soon after the muscle begins to fonn, collagenous fibers arise, and at 

 a still later stage elastic fibers develop. Often in a single protoplasmic 

 mass connective tissue fibers and myofibrillse differentiate side by 

 side. 



As the myofibrillffi form in the protoplasmic syncytium, they tend 

 to run in longitudinal bundles, but usually they show anastomoses 

 with neighboring bundles. Throughout development, and in most 

 instances in the adult pig, the syncytial structure persists. 



The muscle of the digestive tract of man and of chick and of the 

 bloodvessels of chick undergoes a histogenesis very similar to that of 

 the digestive tract of the pig, though in both these forms the syncytial 

 arrangement in later development is not so apparent. 



In general it may be said that complete unifonnity in the structure 

 of adult smooth muscle in different forms and even in the different 

 organs of the same fonn does not exist. Adult smooth muscle may 

 show one of two types and possibly three types of structure, McGill 

 (2), 1907. In type 1 there is very distinct syncytial arrangement 

 with both end and side anastomoses of the fibers, text Fig. 1, which 

 is a persistence of embryonic condition. In Type 2 the muscle 

 bundles show few side anastomoses, but end to end union exists 

 either with or without terminal branching of the fibers, text Fig. 2. 

 (3) There is possibly a third type of smooth muscle. In this type 

 there are apparently no protoplasmic connections between the fibers. 

 Each seems to be an independent spindle-shaped cell, text Fig. 3. 

 Between these three types there are found all transitions. In the 

 description each type will be taken separately. 



Since syncytium as used in recent anatomical writings is a rather 

 indefinite term, its meaning as used in this paper wdll be explained. 

 By muscle syncytium is meant any tissue where there are well defined 

 protoplasmic anatosmoses between the muscle cells. ^Vliere all of 

 tKe cells are so connected the tissue is described as being a complete 



