DEVELOPMENT 



205 



part of the permanent tissues. In certain places the em- 

 bryonic mesoderm cells, hitherto in close contact, separate 

 from one another and become branched, while between them 

 appear intercellular substance and fibres crossing one 

 another in various directions : in this way the connective 

 tissue which form so large a part of the adult body is 

 produced. 



The various parts of the skeleton first arise by the conver- 

 sion of portions of the mesoderm into cartilage. The cells 

 retreat from one another and between them a clear sub- 

 stance, the matrix, makes its appearance. For a considerable 

 time almost the whole skeleton consists of cartilage, but 

 subsequently much of this tissue is replaced by bone 

 developed from a layer of cells on the inner surface of the 

 perichondrium. The bone thus formed in connection with 

 cartilage is cartilage-bone : membrane-bones arise in the 

 connective tissue outside the cartilage (compare p. 43). 



In the place where voluntary muscles are to appear the 

 mesoderm cells elongate, their nuclei multiply by fission, and 

 their protoplasm gradually becomes converted into the 

 striated substance so characteristic of the adult muscular 

 tissue. 



These examples will sufifice to illustrate the fact, which 

 further study would show to be true, that all the permanent 

 tissues are either — like epithelium — formed entirely of actual 

 cells, or are — like connective tissue and striped muscle — de- 

 rived from cells. The entire embryo in its earhest condition — 

 the oosperm stage — is a single cell, which multiplies repeatedly 

 by simple fission, forming a group of cells ; and these, by 

 assuming various forms and undergoing various changes, 

 give rise to all the complex tissues — differing from one 

 another both structurally and functionally — of the adult 

 animal. So that every cell, fibre, or what not in the frog is 



