xii DEVELOPMENT 205 



muscle, as well as to other parts i.e., to by far the greater 

 portion 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 

 appears intercellular substance with fibres crossing one 

 another in various directions : in this way the connective- 

 tissue which forms 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 bones thus formed in connection with 

 cartilage are replacing bones : investing bones arise in con- 

 nective-tissue which has no direct relation to 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 suffice 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 earliest condition 

 the oosperrri 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 



