Late Development

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Development continues with organs becoming more complex and functional. Here is a comparison of early and later stage embryos. Notice the expansion of the chorion in the older embryo. Again, it is possible that the chorion expands because of osmosis due to the buildup of metabolic waste within the egg.

The chorion was removed from some of the eggs to better observe the embryo. The siphon is now nearly formed, and soon the edges will fuse medially to form a hollow tube. Two rows of suckers are visible on the lengthening tentacles. The optic ganglion can be seen in the eye stalks behind the eyes themselves. They eyes are now bright orange and the intricate structures that make up the organ are more clear. The internal yolk sac has retreated from the eyestalks and forms a thick stalk with a ‘Y’ shaped bud projecting into the mantle cavity.

The newly formed brachial hearts and developed gills can be seen within the mantle. The squid has three hearts—two brachial hearts which pump blood through the gills and a systemic heart which moves blood through the rest of the body. Notice the rhythmic pumping of the two brachial hearts in the video. The systemic heart lies between them.

> Over the next several days, the squid has developed chromatophores, which are the red-orange pigment spots on the mantle and head of the embryo. The eyestalks have been reduced. The embryo is more active now and frequently shifts within the egg. It has started to contract its membrane and move its siphon around; behaviors that will help it swim and maneuver once it has hatched.

> Several days later, the eyestalks have disappeared and the mantle and tentacles have lengthened. The embryo now bears much more of a resemblance to the adult. Chromatophores have appeared all over the body and can be seen to contract and expand. Changing the size of these spots of pigment allows the adult squid to turn from a transparent white to an opaque red-brown. The musculature involved in controlling the size of the spots is visible with fluorescent staining.

The branching gills can now be clearly seen within the mantle. Between them, in the center of the mantle, is the dark black ink sac. The rectum lies ventral to the ink sac and buds into two ear-like projects called the anal valves on either side of the sac. The visceral ganglion and otocysts sit dorsally to the opening of the siphon.
The posterior lobes of the internal yolk sac are visible at the very top of the mantle, beneath the two fins. The external yolk sac is very reduced at this point. The embryos continue to be very active at this stage and often push the anterior end of their mantle against the chorion. Between the two fins sits the Hoyle’s organ, a gland which will help dissolve the chorion and egg capsule so that the embryo can hatch (Fields, 1962).

> The squid have begun to hatch at this point. The hatching of the paralarvae from one particular capsule was staggered over several days. The juvenile emerge from a small slit in their chorions with powerful contractions of their mantle. Once free of their eggs, the juvenile squid rise quite quickly towards the surface, angling their mantles upward and their heads downward. They are able to move extremely rapidly in any direction. For the first several days after hatching, the paralarvae sustain themselves with the remains of their internal yolk sacs.

In this video, the juvenile squid can be seen expanding and contracting their chromatophores.