further study by these chapters on human embryology. Kollmann's work
is commendable for its clear treatment of the subject and very fine
original illustrations; its author adheres firmly to the biogenetic
law, and uses it throughout with considerable profit. That is not the
case in Oscar Hertwig's recent Text-book of the Embryology of Man and
the Mammals [translations 1892 and 1899] (seventh edition 1902). This
able anatomist has of late often been quoted as an opponent of the
biogenetic law, although he himself had demonstrated its great value
thirty years ago. His recent vacillation is partly due to the timidity
which our "exact" scientists have with regard to hypotheses; though it
is impossible to make any headway in the explanation of facts without
them. However, the purely descriptive part of embryology in Hertwig's
Text-book is very thorough and reliable.

A new branch of embryological research has been studied very
assiduously in the last decade of the nineteenth century--namely,
"experimental embryology." The great importance which has been
attached to the application of physical experiments to the living
organism for the last hundred years, and the valuable results that it
has given to physiology in the study of the vital phenomena, have led
to its extension to embryology. I was the first to make experiments of
this kind during a stay of four months on the Canary Island,
Lanzerote, in 1866. I there made a thorough investigation of the
almost unknown embryology of the siphonophorae. I cut a number of the
embryos of these animals (which develop freely in the water, and pass
through a very curious transformation), at an early stage, into
several pieces, and found that a fresh organism (more or less
complete, according to the size of the piece) was developed from each
particle. More recently some of my pupils have made similar
experiments with the embryos of vertebrates (especially the frog) and