xn . DEVELOPMENT 205 



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 of the adult 

 body is produced. 



The various parts of the skeleton first arise by the 

 conversion of portions of the mesoderm into cartilage. 

 The cells retreat from one another and between them a 

 clear substance, the matrix, makes its appearance. For 

 a considerable time almost the whole skeleton consists 

 of cartilage, but after metamorphosis 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 connective-tissue which has no 

 direct relation to the cartilage (compare p. 43). 



In the place where the voluntary muscles are to appear 

 the mesoderm cells elongate, their nuclei multiply by 

 fission, and their protoplasm gradually becomes con- 

 verted 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 perma- 

 nent tissues are either like epithelium formed entirely 

 of actual cells, or are like connective-tissue and striped 

 muscle derived from cells. The entire embryo in its 

 earliest 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 a linear descendant, 

 through repeated fission, of the oosperm ; and the 

 oosperm itself is the product of the fusion of two cells, 



