232 



BIOLOGICAL PHYSICS 



This developmental change, marks a great advance, on 

 the preceding stages of embryonic evolution, and allows of 

 the commencement of those further structural arrangements, 

 which permit of the addition of the greatest materio- 



nfe 



FIG. 93. SECTIONS OF THE OVUM OF THE RABBIT DURING THE LATER 

 STAGES OF SEGMENTATION, SHOWING THE FORMATION OF THE 

 BLASTODERMIC VESICLE. (E. v. Beneden.) 



a, section showing the enclosure of entomeres by ectomeres except at one spot the 

 blastopore ; b, more advanced stage in which fluid is beginning to accumulate 

 between entomeres and ectomeres, the former completely enclosed ; c, the fluid 

 has much increased, so that a large space separates entomeres from ectomeres 

 except at one part ; d, blastodermic vesicle, its wall formed of a layer of ecto- 

 dermic cells, with a patch of entomeres adhering to it at one part ; z.p , ecr., enl., 

 as before. 



dynamic change of all, a change in virtue of which, the 

 whole embryonic structures become innervated by two 

 related, independent, and co-ordinated, nervous systems, 

 in addition to that unicellular innervation, which has con- 

 tinued to do duty since the unicellular amalgamation of 

 the parental protoplasms, and which still continues sub- 



