Embryonic shells

Figure IOA Comparison of main morphological features of linguloid embryonic shells from the Early and Late Devonian, Late Cretaceous and Recent (partly after Balinski 1997, modified). The inset shows a hypothetical reconstruction of the embryo of L/ngu/o rugosa Kozlowski from the Early Devonian in dorsal and lateral views.

Hohn, L., Blomqvist, A., and Brandt, I. et al. (2006). Embryonic exposure to o,/ -DDT causes eggshell thinning and altered shell gland carbonic anhydrase expression in the domestic hen. Environmental Toxicology and Chemistry 25, 2787-2793. 

Ca2 + also may be made available from the biocrystalline layer of the shell by changing the pH in a microenvironment. Ca2+ could be absorbed for the construction of the embryonic skeleton. 

The structure, ultrastructure and formation of the hymenolepidid egg has been reviewed in detail by Ubelaker (888). Its general morphology is shown in Fig. 7.14). Although there are only the usual three basic embryonic membranes (p. 179) in the developing egg - shell/capsule, outer envelope, and inner envelope - the fully formed egg often appears to be more complex due to further differentiation of these layers. The following structures can be recognised (Fig. 7.11). 

The presence of a subshell membrane has been noted by a number of workers (439, 440, 541), but it is often difficult to see and its embryonic origin is unknown. In H. diminuta, it is probably represented by the cytoplasmic layer (zone I see below). This membrane appears to be relatively impermeable to many substances and is unaffected by proteases, carbohydrases or lipases. It appears to be a mucopolysaccharide-protein complex and may be important in preventing premature hatching, as well as providing back up protection for the egg shell against a hostile external environment. 

SUBGROUP II.ii FORMATION OF THE TAENIA-TYPE EGG This type of egg is formed essentially in the same way as that of the previous group, only one vitelline cell becoming associated with the fertilised ovum. The embryonic capsule is very thin and almost invisible when laid it is normally lost in faecal eggs. As pointed out earlier, lack of protection by an egg shell is compensated for by the development of a thick embryophore made up of keratin blocks (Fig. 7.1(c)) held together by a cementing substance (442). This gives the egg its characteristically radially striated appearance. 

Fossil shells recognizable as gastropods and bivalves are present in rocks from the Cambrian period, about 570 million years ago. Present classifications based on the evolutionary relationships of mollusks are derived from studies of embryonic development, comparative anatomy, and RNA nucleotide sequences. The findings suggest affinities of mollusks with sipunculid, annelid, and echiurid worms. 

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