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Cover

The composite image on the cover shows, in the background, a scattering of ovoid embryos of the squid Loligo pealeii; each is encased in a chorionic membrane (about 25 m thick all around; 45-50 m at the micropyle, where sperm enter). The size of these early embryos is 1.6 x 1.0 mm, and they have been developing for about 24 hours since their fertilization. The mature eggs from which they developed were fertilized in a petri dish and, shortly thereafter, the egg cytoplasm streamed toward the animal pole and formed a clear lenticular cap called the blastodisc, which underwent meroblastic cleavage, as in birds. The blastodisc is clearly visible as a low, flat projection at the end of the embryo under the micropyle. Also shown on the cover, in the foreground, are two mature 21-day embryos, or hatchlings, one still in its chorion. In life, the hatchlings (in or out of the chorion) would be about the same size (2 mm); the chorion swells to accommodate the growing embryo. [The cover images were produced by Karen Crawford, St. Mary's College of Maryland.]

The embryos on the cover are unusual in that they were cultured in vitro; that is why they are all separate and clean. In nature, squid eggs are released from the female's oviduct in batches of about 180, packaged in elongated, jellylike capsules, or egg strings. Fertilization and development occur within the egg string, which is deposited, with those of other females, in a communal egg mass attached to a suitable benthic surface. The reproduction, reproductive behavior, and development of Loligo pealeii are set out, online, at http://www.mbl.edu/publications/Loligo/squid.

Embryos within egg strings are readily cultured; moreover, they can be snipped out of their matrix periodically and examined, providing a means of following and describing squid development. But the development of embryos that are removed from the egg string soon fails, so the ability to manipulate an early embryo and then to culture it through to hatching is precluded. Thus, many methods of experimental and comparative embryology become difficult or impossible with squid: e.g., the effect on later stages of manipulating earlier ones; classical chemical treatments that perturb axis formation; isolation of large numbers of specific stages of embryos for molecular analysis; and even time-lapse microscopy. The result is that squid embryos, being difficult to work with, have been neglected.

In the summer of 1984, at the General Scientific Meetings of the Marine Biological Laboratory, Karen Crawford (Klein) and Laurinda A. Jaffe described a method of fertilizing squid eggs in vitro and culturing them through organogenesis to chorionated hatchlings. Now, 17 years later and at the same venue, Crawford shows us that the embryos can be made to hatch on their own. More important, she reports (p. 251) that treatment of fertilized eggs of Loligo pealeii with colchicine, but not cytochalasin D, interferes with ooplasmic segregation and blastodisc formation, suggesting that microtubules participate in these processes--in contrast to the process as it is known in zebrafish.

This short report is signaling that squid embryogenesis is now accessible and may be applicable and informing to other aspects of physiology currently being studied with hatchlings or adult animals. Some of these aspects are represented in this issue: e.g., neuronal development (J. P. H. Burbach et al., p. 252); morphological and functional ontogeny of squid mantle (J. T. Thompson and W. M. Kier, p. 136; p. 154); polarization patterns in squid and cuttlefish skin (N. Shashar et al., p. 267); vesicle transport in giant axon (J. R. Brown et al. [p. 240] and J. R. Clay and A. M. Kuzirian [p. 243]); and excitability (J. R. Clay and A. Shrier, p. 186).

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