ch. iv.] THE LAW OF EVOLUTION. 341 



the outer and inner portions of the yolk-mass : the outer 

 cells become flattened and pressed together, so as somewhat 

 to resemble a mosaic pavement, and thus form a peripheral 

 membrane. As this membrane continues to thicken by the 

 integration of adjacent materials, it differentiates into two 

 layers, wrapped the one within the other, like two coats of 

 an onion. The outer layer, or ectoderm, absorbing larger 

 quantities of nitrogenous matter than the other, is the one 

 which by further immense differentiation is destined to 

 produce the bony, muscular, and nervous systems ; while 

 the inner layer, or endoderm, is destined to produce the 

 digestive apparatus. Between these two, by a further 

 differentiation, arises a vascular layer, the rudiment of the 

 circulatory system. Now on the interior surface of the 

 endoderm appears a grooved channel, of which the edges 

 gradually rise and fold over towards each other until joining 

 they form a tube, — the primitive alimentary canal. At first 

 nearly uniform, this channel becomes slowly more and more 

 multiform. Near the upper end it bulges so as to form a 

 stomach, while the long lower portion, variously wrapped and 

 convoluted, is differentiated into the small and large intestines. 

 From various parts of the now heterogeneous canal, there 

 bud forth variously-organized secreting glands, — those which 

 make saliva, and those which make gastric juice, bile-cells, 

 pancreatic cells, and intestinal follicles. While from the 

 exterior coat of the endoderm, thus wonderfully transformed, 

 there shoot out, near the 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 capillaries. " 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." Presently 

 the auricle, or chamber which receives blood, is differentiated 

 from the ventricle, or chamber which expels it ; and still later 



