From Germ layers to Cells

As surprising as it may seem, it was not always known that cells are the basic units that make up all living things. Rather, improvements in microscope technology in the early nineteenth century along with rising support for the idea that there exists a fundamental unit of life led Matthias Schleiden and Theodor Schwann to establish cell theory in the late 1830s. While the initial theory was later modified, such as by Rudolf Virchow’s addition that all cells arise from preexisting cells, it had tremendous impact on the way biologists at the time thought about the organization of life and, in the case of embryologists, its beginnings. By the 1870s, the field of cytology was well established and attracting the attention of many leading biologists of the day (Maienschein 1991).

In embryology, equally influential as cell theory was the concept of the germ layers, which ultimately gave rise to the establishment of germ layer theory in the late 1860s (MacCord 2013). First introduced by Karl Ernst von Baer and Christian Heinrich Pander in the beginning of the nineteenth century and further developed over the next few decades, the concept of germ layers had a major influence on how embryologists of the time made sense of embryonic organization and development. Work published by Aleksandr Kovalevsky in 1867 catalyzed the establishment of germ layer theory shortly thereafter (MacCord 2013). The germ layer theory held the germ layers as homologous structures, meaning that each type of layer results in the same adult tissue types and organs in all species (MacCord 2013).

Arising through a combination of cell and tissue movements known as gastrulation, the germ layers are layers of tissue constituting the early embryo that will ultimately give rise to specific structures and systems within the adult body. The ectoderm generates the outer layer of the embryo and will give rise to the epidermis and nervous system while the endoderm (which Wilson calls the entoderm) generates the innermost layer and will become the lining of the digestive tract. Sandwiched between the ectoderm and endoderm in the embryo is the mesoderm, which gives rise to muscles, gonads, and other connective tissues (Gilbert 2014).

Although cell theory and germ layer theory were worked out and established somewhat concurrently, cytology wasn’t incorporated into the study of early embryonic development until figures such as E. Ray Lankester and Charles Otis Whitman began closely studying cell divisions in the developmental stages prior to the establishment of the germ layers (Maienschein 1991). This new approach is what prompted individuals like Wilson and Edwin Grant Conklin to begin carrying out cell-lineage studies in the late 1880s. For Wilson, another motivation for turning his attention to cells was the persisting deadlock in opinion within the field regarding the establishment of the germ layers as the beginning of developmental events and their ability to shed light on evolutionary homologies among organisms (Wilson 1892). Through his cell-lineage studies, Wilson sought to expand the germ layer theory to include early developmental events prior to gastrulation, such as the very first cell divisions.