274 RAMBLES OF A NATURALIST. 



more than a sort of membranous disc hollowed out Into 

 irregular lacunas. These may be regarded, according 

 to M. Milne Edwards, as so many small lakes commu- 

 nicating with one another by means of tortuous canals. 

 In proportion as the organisation of the embryo 

 advances, these canals become enlarged, the lakes 

 change into streams, and the canals which had at 

 first been simply hollowed out of the actual substance 

 of the tissues, becoming enclosed, are invested with 

 a tubular membrane, and thus pass into the condi- 

 tion of vessels properly so called. Precisely simi- 

 lar phenomena occur in the false membranes which 

 often supervene on an inflammatory affection of the 

 thoracic organs. Here also the plastic matter 

 becoming organised under the abnormal influence of 

 excessive vitality, gives rise to lacunas, which becom- 

 ing converted into vessels, are soon brought into 

 communication with some of the pre-existing branches 

 of the circulating system. In the presence of such 

 an array of facts, derived from wholly different 

 sources, it seems scarcely unreasonable to think that 

 the same order of things may be generally present, 

 and that usually, if not always, a lacuna has preceded 

 a vessel. 



Such, indeed, was the conclusion at which M. Milne 

 Edwards arrived in opposition to the cellular theory 

 of Schwann, one of the most distinguished pupils of 

 the celebrated M tiller.* According to the German 

 physiologist, all parts of the animal body have been 

 primarily composed of simple cells. This universal 



* [A sketch of the scientific labours of Professor Miiller is given 

 in the Appendix, Note XVI.] 



