THE LAW OF EVOLUTION 



niteness of the resulting structures. " In the 

 mammalian embryo, the heart, at first a long 

 pulsating blood-vessel, by and by twists upon 

 itself and integrates. The bile-cells constituting 

 the rudimentary liver do not simply become 

 different from the wall of the intestine in which 

 they at first lie ; but as they accumulate, they 

 simultaneously diverge from it, and consolidate 

 into an organ. The anterior segments of the 

 cerebro-spinal axis, which are at first continuous 

 with the rest, and distinguished only by their 

 larger size, undergo a gradual union ; and at 

 the same time the resulting head folds into a 

 mass clearly marked off from the rest of the 

 vertebral column. The like process, variously 

 exemplified in other organs, is meanwhile ex- 

 hibited by the body as a whole — which be- 

 comes integrated somewhat in the same way 

 that an outspread handkerchief and its contents 

 become integrated when its edges are drawn in 

 and fastened to make a bundle." Mr. Spen- 

 cer, from whom I have quoted this embryo- 

 logic illustration, goes on to cite parallel in- 

 stances in the development of lower forms of 

 animal life ; a few of which may be here epi- 

 tomized. In the growth of the lobster from 

 its embryo, a number of calcareous segments, 

 originally separable, become integrated into the 

 compact boxes which envelop the organs of 

 the head and thorax. A similar concentration 

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