380 THE LAW OF EVOLUTION CONTINUED. 



outspread on the yelk-sac, gradually rises up from it, 

 and by the infolding of its ventral region, becomes a sepa- 

 rate mass, connected with the yelk-sack only by a narrow 

 duct. 



These changes through which the general structure is 

 marked out with slowly- increasing precision, are paralleled 

 in the evolution of each organ. The heart begins as a 

 mere aggregation of cells, of which the inner liquefy to 

 form blood, while the outer are transformed into the walls; 

 and when thus sketched out, the heart is indefinite not only 

 as being unlined by limiting membrane, but also as being 

 little more than a dilatation of the central blood-vessel. 

 By and by the receiving portion of the cavity becomes dis- 

 tinct from the propelling portion. Afterwards there be- 

 gins to grow across the ventricle, a septum, which is, how- 

 ever, some time before it shuts off the two halves from each 

 other; while the later-formed septum of the auricle remains 

 incomplete during the Avhole of foetal life. Again, 



the liver commences by multiplication of certain cells in the 

 wall of the intestine. The thickening produced by this 

 multiplication " increases so as to form a projection upon the 

 exterior of the canal; " and at the same time that the organ 

 grows and becomes distinct from the intestine, the channels 

 running through it are transformed into ducts having clear- 

 ly-marked walls. Similarly, certain cells of the external 

 coat of the alimentary canal at its upper portion, accumulate 

 into lumps or buds from which the lungs are developed; 

 and these, in their general outlines and detailed structure, 

 acquire distinctness step by step. 



Changes of this order continue long after birth; and, 

 in the human being, are some of them not completed till 

 middle life. During youth, most of the articular surfaces 

 of the bones remain rough and fissured the calcareous 

 deposit ending irregularly in the surrounding cartilage. 

 But between puberty and the age of thirty, these articular 

 surfaces are finished off into smooth, hard, sharply-cut 



