Epiboly in Fundulus occurs when the enveloping layer of the blastoderm thins and spreads over the YSL and the yolk cell. As mentioned above in Chapter 6, "Genesis of a Research Problem", Warren Lewis had earlier proposed that epiboly occurs when the surface gel of the YCL contracts and exerts force on the blastoderm, initiating its movement. Trinkaus, in contrast, hypothesized that the YCL did not contract during epiboly; rather the YCL was engulfed by the YSL during epiboly, and the YSL could trigger the process of epiboly separately from the blastoderm (Kimmel & Weston 2003, 587; Trinkaus 1951). To support his hypothesis, Trinkaus surgically removed the blastoderm from the yolk cell – a procedure that requires considerable skill in microsurgery – and observed that the YSL continued to undergo epibolic movement without the blastoderm.
In subsequent work, Trinkaus discovered the structural features of the YSL, such as its actin-based cytoskeletal meshwork and its adhesive junctions, which enable it to perform its endocytic functions, engulfing the YCL, and to pull along the blastoderm during epiboly (Kimmel & Weston 2003, 587; Betchaku & Trinkaus 1986). These experiments allowed Trinkaus to make considerable progress in understanding the structure and function of the components of the early embryo. His early work on Fundulus epiboly established the importance of the YSL in the process of gastrulation, and it remains the accepted model of epiboly
Trinkaus also established that the blastoderm can move independently from the YSL during epiboly due to its deep cells that drive its locomotion. However, he didn’t think that the deep cell movements of the blastoderm were essential to the process of epiboly; they were merely an effect of the coordination of the deep cells forming the embryo (Trinkaus 1984).
Trinkaus’s work on Fundulus epiboly established the importance of the YSL in the process of gastrulation, and it remains the accepted model of epiboly in developmental biology. Trinkaus later studied the origins of the YSL, as well as the cellular mechanisms that could explain the locomotion of deep cells. He conducted most of this research during the summers in his lab in Whitman 210 at the MBL.