THE LAW OF EVOLUTION 



upper end, little flower-like buds, which by 

 and by become lungs. In the intermediate or 

 vascular layer, equally notable differentiations 

 simultaneously occur. The vascular channels 

 become distinguished as veins, arteries, and cap- 

 illaries. " The heart begins as a mere aggre- 

 gation of cells, of which the inner liquefy to 

 form blood, while the outer are transformed into 

 the walls." Presently the auricle, or chamber 

 which receives blood, is differentiated from the 

 ventricle, or chamber which expels it ; and still 

 later a partition-wall divides first the ventricle 

 and afterwards the auricle into two portions — 

 one for the venous, the other for the arterial 

 blood. Along with all these changes, parallel 

 processes, too numerous to be more than hinted 

 at, are going on in the ectoderm. Masses of 

 nitrogenous cells here give rise to muscles, 

 which ramify through the whole interior of the 

 embryo ; and there to cartilaginous structures, 

 in which deposits of earthy phosphate, harden- 

 ing around certain centres, generate bone. The 

 nervous system, first appearing as a mere groove 

 upon the surface of the germinal membrane, 

 finally exhibits an almost endless heterogeneity. 

 First there is the difference between gray and 

 white tissue, of which the first generates the pe- 

 culiar kind of molecular motion vaguely termed 

 nerve-force, while the latter transmits such mo- 

 tion. Then there are the differences between 

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