The mammalian egg. Since de Graaf, the large follicle in the ovary was regarded as the egg, but many scholars doubted his results. They wondered, among other things, how such a big object could pass through the narrow tubes. In the mid-1820s, Karl Ernst von Baer was certain that the egg must originate in the ovary but that it was not the follicle de Graaf had observed. He had his teacher and colleague Karl Burdach give him a house bitch that had gone into heat a few days earlier. Seeing only follicles in the ovaries, Von Baer was about to give up, when with the unaided eye he noticed ‘a yellow spot’ in some of them. Opening one, he lifted the spot into a watch glass for microscopy. He had hoped so much for this sight, but when it finally came he recoiled ‘as if struck by lightning, … afraid I had been deluded by a phantom’. The discovery of a tiny elementary structure, from which a whole organism developed, was an important inspiration for the later cell theory.

Cells and disciplines

Microscopy led to the discovery of cells and embryology was institutionalized in medical courses.

Researchers in German institutes of anatomy and physiology followed Pander and von Baer. After the discovery of the true mammalian ovum, a more general cell theory was developed from the late 1830s. In the early 1850s, Robert Remak, whose Jewish faith hindered his career at the University of Berlin, argued that all cells arise from pre-existing cells, from the egg via the germ layers to the tissues. He also taught that in vertebrates each layer gives rise to particular cell types, for example, liver from endoderm, muscle from mesoderm and nerve from ectoderm.

By the 1850s special anatomical courses for medical students were embryology’s main base. Developmental approaches were very important, but the science achieved only limited institutional independence. A new, physics-oriented physiology reckoned embryos intractable with physico-chemical methods and abandoned the subject to anatomy and the newly independent zoology institutes. Few human specimens from the first month and none from the first two weeks were known, but by teaching that vertebrate development follows a common pattern, comparative embryology licensed the use as surrogates of frog, chick and domestic mammals.

An optician’s workshop, 1832


Lithograph of cleaving dog embryos, 1845