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Early Development

Each egg mass consists of groups of eggs contained within a jelly matrix and leathery, translucent outer membrane layers. Most of the outer layers were peeled off to better observe development within the capsules. On average, the egg capsules are 8 cm long, with one end forming a stalk which is attached to the sea floor (Fields, 1962). As the eggs matured and grew larger, the capsule was stretched in length.

The eggs themselves contain a large ball of yolk surrounded by a transparent chorion. Market squid are lecithotrophic, meaning that as they develop, they feed on yolk within their eggs rather than moving about the water column to feed on plankton (Allen and Pernet, 2007).  This strategy can be costly for the mother as she must provide her embryos with all the nutrients they will need to develop.  On the upside, the squid embryo does not need to expend additional energy feeding and undergoes an abbreviated larval period compared to species which must seek out more of their food (Allen and Pernet, 2007). A portion of the yolk will remain inside the embryo throughout its development and even several days after hatching (Fields, 1962). Market squid, like other cephalopods, exhibits direct development and does not pass through an intermediate larval stage before assuming the adult body plan. Market squid hatch out of their eggs looking like miniaturized adults.

The embryo begins formation through epiboly, a method of gastrulation where cells at the top of the yolk mass migrate around to the opposite pole, encapsulating the yolk with a layer of cells in the process (Warga and Kimmel, 1990). Cephalopods, unlike other mollusks, do not exhibit spiral cleavage (Brooks, 1880; Wadeson and Crawford, 2003). Cleavage in the market squid is meroblastic, meaning early cell division is partial and sister cells remain connected to one another rather than completely separated. Early division occurs at one end of the yolk mass, forming a cap of cells called the blastodisc (Fields, 1962; Arnold et al., 1974). Much of organogenisis occurs at the blastodisc.

> Several days after we received the eggs, the blastoderm began to differentiate into new structures and bilateral symmetry was developed.

The circular, raised cap on the end of the embryo will give rise to the mantle in the adult animal. The two bulges beneath the developing mantle are the rudimentary eye-stalks, and in their center the early stages of the eye can be seen.

Here, bilateral symmetry can be seen as the embryo rotates within the chorion. The embryo is ciliated and often moves (Brooks, 1880). The space within the chorion is filled with a cloudy fluid. This is suggested to be metabolic waste that aids in the expansion of the chorion through osmotic pressure (Brooks, 1880; Fields, 1962).

> The shape of the forming embryo is even more distinct by the following day. The rudimentary arms of the squid can be seen as a ridge or little projections around the center of the embryo beneath the eye stalks, which bulge out more distinctly. The anteroposterior axis is clearer now, with the mantle forming at the posterior end of the animal and the eyes forming at the anterior. The embryo continues to rotate within the chorion.

> The next day, the mantle overhangs the body on both sides and the eye stalks are much larger. The projection of the yolk into the mantle and eye-stalks can be seen clearly. The rudimentary arms are even more distinct at this stage. Newly visible are the posterior and anterior siphon-folds, which will eventually form the siphon in the adult animal. The siphon-folds are seen as paired, symmetrical ridges of tissue in the center of the embryo between the eye stalks. The presence of the siphon indicates that this will be the ventral side of the adult animal.
Also just visible are the otocysts or statocysts; paired, donut-shaped ridges of tissue which function in orientation. These organs lie at the posterior end of the anterior siphon folds.

> By the following day, the embryo is much more complex. The arms have elongated and the mantle and mantle cavity (where the gills are forming) are even more distinct. Rudimentary fins can be seen on the surface of the mantle.  The rectum is now present as a raised hollow rod and the free edges of the two anterior siphon folds have bent towards each other to begin the formation of the siphon opening. Eventually the anterior and posterior siphon folds will meet to form a funnel, and the mantle will descend over top of the posterior section. The eye stalks are becoming more defined and the yolk protuberances no longer entirely fill them. The optic ganglia are beginning to form in the space cleared by the retreating yolk.

If you look closely, you can see peristaltic contraction in the constricted portion of the yolk where it meets the body. These contractions play a part in circulating nutrients through the embryo (Fields, 1962).

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