88 



HISTOLOGY 



THE HISTOGENESIS OF STRIATED MUSCLE 



Most striated muscle is developed from some sort of external or 

 internal epithelium, the fibrils being formed in situ in the periphery or 

 on one side of a columnar or prismatic cell, which may have many posi- 

 tions in the body. 



In the medusa it is the external, covering epithelium that develops the 

 myo-fibrils. These fibrils are formed in the expanded bases of the cells. 



The cells may lie on the surface 

 or they may be invaginated into 

 grooves, and only the inner cells 

 of the groove have the muscle 

 characteristics, as in Figure 85. 

 This represents a transection of 

 three such invaginated grooves in 

 the epithelium on the upper sur- 

 face of the body of a Florida me- 

 dusa, Cassiopea xamachana. It 

 will be noticed that only the one 

 or two cells lying in the bottom 

 of the groove (which is closed) 

 have developed the muscle fibrils, 

 which may be seen in section as 

 black dots. In a younger speci- 

 men or on the outer and weaker 

 surfaces of this same specimen 

 there would be no invagination, 

 and the myo-fibrils would be seen lying in the- bases of nearly all the 

 cells. 



The vertebrate animals develop their muscle in cells that were origi- 

 nally a part of the epithelium that lined the primitive ccelom. These 

 cells lose their connection with the ccelom when the epithelium, of which 

 they are a part, is invaginated from the ccelomic surface into a number 

 of buds, which are cut off and form a row of myotomes or myomeres hi the 

 sides of the animal. These sac-like masses flatten in a manner to form 

 two plates and reduce the invagination cavity to a plane line. The inner 

 plate forms connective-tissue elements, while the outer develops the 

 striated, voluntary muscle of the body. 



The development of striated muscle in the embryo fish will furnish us 

 with a good example. We shall use an embryo sucker, which is easily 

 procured and prepared. 



A section of a very young embryo of about 2 mm. shows the 



mut.f?" 



FIG. 85. Muscle cells derived from ectoderm 

 in Cassiopea xamachana. mes.c., mesodermal 

 cell; ect.c., outer ectodermal cells, which prob- 

 ably play but small part in muscle formation ; 

 ntus.c., ectodermal cells which are invaginated 

 into grooves and form muscle fibrils in their 

 proximal cytoplasm. The grooves and mus- 

 cle fibrils (mus.f.) are both seen in transverse 

 section. 



