THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 313 



In some cases, for example in the muscular layers of the gastrointestinal 

 tract, distinct bands or sheets of smooth muscle are formed in which the cells 

 are closely packed and lie approximately parallel. In other cases, such as the 

 mucosa of the intestine and the capsules of certain glands, the muscle cells 

 develop in little groups or as isolated cells. 



Anomalies. 



More or less of the muscular system is involved in some of the gross anoma- 

 lies or malformations of the body. For example, congenital defects in the cen- 

 tral nervous system (anencephaly, rachichisis, etc.) are necessarily accompanied 

 by atrophy or faulty development of certain parts of the muscular system. In 

 the case of ventral median fissure of the abdominal wall (gastroschisis), the 





FIG. 281. From a section of developing heart muscle in a rabbit embryo of 10 mm. Godleuvski, 

 The fibrils are segmented, indicating the beginning of the cross striation characteristic of heart muscle. 



abdominal muscles are naturally involved. Such anomalies in the muscles are, 

 however, secondary to the other malformations and are not due to primary 

 defects in the muscles themselves. 



Many of the minor variations in the muscular system occur in the same 

 form or in similar forms in different individuals, thus indicating their relation to 

 a fundamental type. Many of these are more or less accurate repetitions of 

 normal structures found in lower animals. Such variations are probably 

 rightly attributed to hereditary influences. On the other hand, there are varia- 

 tions which cannot be referred to conditions found in any of the lower animals. 

 These constitute a class of variations which must be accounted for upon some 

 other basis than that of heredity. As pointed out in the chapter on Teratogene- 

 sis (Chap. XIX), external influences undoubtedly play an important part in the 

 production of anomalies and it is probable that similar influences act upon the 

 development of the muscular system. 



The scope of this book does not permit a description, or even mention, of the 

 great number of variations in the muscles. A few are described here as ex- 



