Embryonic Development and Induction

95 gestion that viruses, from the type which appears to be of molecular proportions to those which are essentially microorganisms in the bacterial sense-have one (at least) distinguishing characteristic in common,-parasitism. From this he deduces: "We would suppose that the larger viruses were organisms which had lost the power to synthesise some factor or factors essential for their growth and multiplication, perhaps a ferment and co-ferments. As we pass down the scale and the viruses diminish in size, we would postulate the loss of more and more factors essential for growth. The intermediate sized viruses would have lost several essential ferment systems, and... the smallest viruses would have lost all ferments and all auto-synthetic potentialities. .. Divorced from the particular cells which they can infect, the structures we postulate would be inert and in a state of suspended animation; within the correct cell, however, they would have at hand all substances required for growth, and they would attract to themselves the requisite ferment systems from the host cells or utilise those in their immediate neighbourhood." A thesis "that viruses are decadent forms of organisms degraded through long persistent parasitism," and that only, offers opportunity for thought, and may well provoke discussion. An able cytologist and his daughter have assembled a series of short descriptions of well-selected histological methods and technics. Many of these methods have been devised, developed, or adopted by Dr. Bensley during his long years of studies on the cell. Although some parts contain advice purely to beginners, the book has a useful place in any laboratory of histology. The procedures for studying living cells are well selected and should be included in any good course in Microscopic Anatomy. The book contains an index and one page of good references to literature on the subject. In these, the Silliman Lectures, Dr. Hans Spemann, one of the outstanding embryologists of the world, presents the results of years of experimentation carried out by himself and his students. These experiments were devised to answer numerous questions regarding the factors which determine and control development. Using the methods of experimental embryology, alterations were made in the relationships between parts of developing embryos