At first, the blebs on the cell surface do not contain any of the cytoplasmic organelles, such as mitochondria or endoplasmic reticulum, because of a barrier formed by actin-rich microfilaments. When cell motility occurs, the blebs continue to protrude from the cell surface, instead of protruding and retracting. This breaks down the filament barrier and allows cells’ cytoplasm and organelles to flow into the extended blebs. At this stage, the extended blebs are referred to as lobopodia because both their form and function have changed sufficiently to produce an identifiable organ of locomotion (Trinkaus 2003, 233). The movement of the cells occurs through extensive cytoplasmic flow through the lobopodia, much like the movement of an amoeba. At this stage these cells can also adhere to the surface of the yolk syncytial layer and form a lamellipodium, which can attach to other cells, contract, and pull the cells forward in a directional movement. According to Trinkaus, this research showed that the process of gastrulation in Fundulus occurred via two stages: First, the cells need to develop the mechanisms for motility, and second, they need to respond to cues that direct their movement during gastrulation to form the embryo. Trinkaus summed it up accordingly: “Before moving directionally, you first have to learn how to move” (Trinkaus 2003, 234). This research was summarized in Trinkaus’s paper, “Surface activity and locomotion of Fundulus deep cells during blastula and gastrula stages,” published in Developmental Biology in 1973.