AUTHOR'S PREFACE. xi 



opinion still maintained its ground, even when the structiuc 

 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 1837, 

 when an actual growth of the elementary particles of epithe- 

 lium was proved to take place without vessels. Henle (Sym- 

 bols 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 1 says (1. c. p. 9), " Hoc in loco (in planta 

 pedis) cellularum (retis Malpighii) diametrum extrorsum 

 augeri, saepius repetita observatione pro re certa affirmare 

 audeo. Quas retis cellulas non minus in fcetu 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 — squamulae — epithelii nondum ita con- 

 formatae sunt ut in ilia periodo, quae partui praecedit, scd 

 parenchyma plantarum cellulis simillimum ostendunt, cum 

 quaeque squamula, qua3 postea talis apparet, tunc temporis 

 tanquam cellula polyedrica e membrana tenacissima const an s 

 globosamque guttulam continens in conspectum veniat. Pressu 

 applicato rumpebantur istse cellules atque lymphaticum liquo- 

 rem effundebant, quae cellulas, procedente evolutione, verisimile 

 complanatae in illas polyedricas squamas ruutantur." Ilenle, 

 when quoting this passage, adds (1. c. p. 9) : " Haec ilia mini 

 vero sola compressio in causa esse possit, ut parva cellula 



1 Henle's observations are detailed at page 7G of this treatise. The research' 

 Turpin and Dumortier could not be quoted, as I only became acquainted with them 

 at the conclusion of my work. 



