342 COSMIC rillWaOPUY [pt. H. 



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, hardening 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 heterogc ne ty. First there 

 is the difference between grey and white tissue, of which the 

 first generates the peculiar kind of molecular motion vaguely 

 termed nerve-force, while the latter transmits such motion. 

 Then there are the differences between the nervous centres 

 which, differently bundled together, make up the cerebrum, 

 the cerebellum, the corpora quadrigemina, the medulla 

 oblongata, the spinal cord, and the sympathetic ganglia, each 

 of which aggregates is extremely heterogeneous in itself. 

 And then there are the innumerable differences entailed 

 by the highly complicated connections established between 

 one nervous centre and another, by the inosculations of 

 different sets of nerves with each other, and by the circum- 

 stance that some nerves are distributed upon muscles, others 

 upon glands, and others upon ganglia. 



These must suffice as examples of differentiation. To go 

 on until we had exhausted the series of differentiations which 

 attend the evolution of a single individual, would be to write 

 the entire history of an organism, and thus to convert our 

 philosophic discussion into a special scientific monograph. 

 That history was long since thoroughly written by Von Baer. 

 Following out hints furnished by Linnreus, K. F. Wolff, 

 Goethe, and Schelling, this illustrious embryologist announced, 

 in 1829, his great discovery that the progressive change 

 from homogeneity to heterogeneity is the change in which 



