Yolk sac morphology in cephalopod embryos

The basic pattern of cephalopod embryogenesis is uniform, whereas the specific ovum sizes vary from less than 1 mm to about 30 mm. The organ-forming part (called the "embryo cap", or the "embryo proper") of the epibolic gastrula covers a larger (in small eggs) or smaller part (in !arge eggs) of the animal hemisphere of the uncleaved yolk mass. The rest of the gastrula forms an envelope for the portion of yolk that remains outside the embryo cap; it constitutes the so-called outer yolk sac, a transient auxiliary organ of the embryo. The overall shape of the yolk mass at early embryonic stages varies among systematic groups of cephalopods, from nearly globular to elongate/oval. At advanced stages of organogenesis, the outer yolk sac becomes increasi ngly distinct d ue to a constriction of the brachial and cephalic zone of the embryo proper; an exception are some teuthoid squids (e.g. the Ommastrephidae), in which the outer yolk sac remains rudimentary. The portion of the yolk mass lying inside the embryo proper is called the inner yolk sac and yolk neck (the latter connects the inner with the outer yolk sac). At advanced embryonic stages, the yolk neck is a simple strand, which is more or less strongly compressed by the organs of the head. The inner yolk sac takes on a shape that is generated by the surrounding organs; they und ergo a progressive concentration leading to a size reduction of the inner yolk sac. A shift of partial pressure between the outer and the inner yolk sac occurring at late embryonic stages appears to support a secondary enlargement of the inner yolk sac, the final shape of which is defined by special morphogenetic processes shaping the whole visceral complex of the embryo. Thus more or less distinctive morphologies of the inner yolk sac are achieved. In a strictly functional perspective, the various yolk sac morphologies can be viewed as different modes of yolk storage during the developmental phase that leads to hatching of the young animal. In all the cephalopods so far studied, the newly hatched young can survive some time without foraging. Und er normal conditions, however, the embryonic nutriment remaining in the inner yolk sac of the hatchling is rapidly (within days or weeks) absorbed in parallel to, but independently of digestive processes that are induced by the capture and ingestion of prey. This coexistence of lecithotrophy and active feeding is due to the morphological and physiological separation of the embryonic (yolk organ) and post-embryonic alimentary systems. The actual duration of this concomitancy is partly conditioned by the water temperature acting on the metabolism, and by the volume of the yolk reserve which in turn depends largely on the yolk storage capacity of the visceral mass