The heritage of experimental embryology: Hans Spemann and the organizer

At a minimum, these errors, omissions, and borrowings raise questions about the depth of Dr Harding's research and the originality of his analysis. I came into embryology as a postdoctoral fellow in 1974, when the subject was rather unfashionable and neglected. Over my first few years I spent long hours in the library and read with fascination of an earlier "golden age" between the two World Wars. Slowly I pieced together the story of the organizer. It took time because most of the papers had to be retrieved from dusty stacks and because most were in German, a language I read poorly. How I would have loved to read Professor Hamburger's book then! The organizer graft is a transplantation of tissue from the dorsal lip of the blastopore of an amphibian gastrula to the prospective ventral lip of another individual. When performed correctly, it yields a mirror symmetrical double dorsal embryo, rather like a pair of Siamese twins joined belly to belly. The lower half of the duplication, often called the secondary axis, consists of a notochord derived from the graft and the remainder, mainly somites and neural tube, is induced from the ventral tissues of the host. The organizer graft was first reported in a famous paper by Spemann and Mangold in 1924. Hamburger was a graduate student in Spemann's department at Freiburg during the years that the organizer grafts were first being performed. In his book he describes the scientific background, the character of experimental work at the time, and something of the personality of the individuals involved. It is certainly sobering to be reminded of the experimental difficulties of the time. In my laboratory today we generate two or three batches of Xenopus eggs each week, we manipulate their developmental rate with incubators at different temperatures, and we protect our grafts and explant cultures from infection with antibiotics. During the 1920s, embryologists had to collect their eggs (usually newt eggs) from the wild during the brief breeding season in the spring. The whole year's experiments would be performed in a mad rush, with horrific mortality rates due to poor culture conditions and to infection. It is because of these difficulties that the famous organizer paper describes only six cases, of which only two show good double-dorsal duplications. Hamburger goes on to describe the subsequent work on early amphibian development. Unfortunately the organizer, which we now regard as …