AUTHOR’S PREFACE. XI 
opinion still maintained its ground, even when the structure 
of the tissues became more accurately known. Nor did the 
plant-like growth of the component parts of the ovum abolish 
the assumed essential difference of the growth of the vascular 
tissues. 
A very important advance was made in the year 1887, 
when an actual growth of the elementary particles of epithe- 
lium was proved to take place without vessels. Henle (Sym- 
bole ad anatomiam vill. intest. Berol. 1837) showed that the 
cells in the superficial layers of epithelium are much more ex- 
panded than those in the deeper strata, a fact which leaves 
scarcely any doubt as to their true plant-like (i. e. non-vascular) 
growth. Henle’ says (l. c. p. 9), “Hoe in loco (in planta 
pedis) cellularum (retis Malpighii) diametrum extrorsum 
augeri, sepius repetita observatione pro re certa affirmare 
audeo. Quas retis cellulas non minus in fceetu suillo sensim 
increscentes transire in cellulas epidermidis, nunquam non 
inveni.” Purkinje and Raschkow (Meletem. circa mammal. 
dentium evol. Vratisl. 1835) had made the following obser- 
vations upon the development of the epidermis: “In primis 
evolutionis periodis—squamule—epithelii nondum ita con- 
formate sunt ut in illa periodo, que partui precedit, sed 
parenchyma plantarum cellulis simillimum ostendunt, cum 
quzque squamula, que postea talis apparet, tunc temporis 
tanquam cellula polyedrica e membrana tenacissima constans 
globosamque guttulam continens in conspectum veniat. Pressu 
applicato rumpebantur istz cellule atque lymphaticum liquo- 
rem effundebant, que cellulz, procedente evolutione, verisimile 
complanatz in illas polyedricas squamas mutantur.” Henle, 
when quoting this passage, adds (l. c. p. 9): “ Hee illa num 
vero sola compressio in causa esse possit, ut parva cellula 
' Henle’s observations are detailed at page 76 of this treatise. The researches of 
Turpin and Dumortier could not be quoted, as I only became acquainted with them 
at the conclusion of my work. 
