298 



EGBERT TRACY JACKSON ON THE 



Dr. Ilorst says that great changes take place during further growth of the embrjo. 

 The preconeh}lian gland gradual!}' loses its primitive character of a glandular invagi- 

 nation and forms a thickening of the ectoderm composed of long, conical cells, fig. 22, si-. 

 A thin, cuticular membrane secreted by these cells is the first indication of the shell, and 

 at this position in the adult the hinge is found.' Horst maintains, therefoi-e, that the 

 shell of the oyster originates as a single plate in accordance with the observations of 

 Brooks in Anodon and Hatschek in Teredo.^ Meanwhile in fig. 22 a body cavity has 

 formed by the separation of the ectoderm and entoderm, a velum has developed and the 

 gastrula invagination has formed a spacious cavity in the base of which is a diveiticu- 

 lum, now closed, but which later communicates with the ectoderm aiid forms the jjoste- 

 rior part of the alimentary canal. The neoembryonic mollusc in our classification is rep- 

 resented ]\y the trochosphere stage before the development of the shell-gland, PI. xxni, 

 figs. 3-4. In the developing European oyster the shell-gland, according to Ilorst, orig- 

 inates early, closely following the gastrula invagination and befoi-e the formation of a 

 normal trochosphere stage. I think, therefore, that we must consider the neoembryo 

 stage as skipped in the oyster by theoverlap[)ing of stages due to extreme concentration 

 and acceleration of development.'* Fig. 22, on the other hand, is the neai'cst apjiroach to 

 the ideal t3"pembr3'o stage that we have in the develo}:)ing oyster (definition of typem- 

 bryo, see p. 291). The primitive gastrula cavity has not yet become connected with the 

 ectoderm by the secondary opening, the anus, as is ordinarily the case, but the existence 

 of the shell-gland, with the beginnings of the velum, entitles it to be considered as a 

 typembryo. The existence of these comparativel}^ high characters .in the young 03^ster, 

 at a period when the primitive oral invagination still retains almost its early simplicity 

 and no second alimentary opening has been formed, must be considered as strong proof 

 of the higlily concentrated development of this type. 



Taking up Horst's account of the developing oyster again, he says that on the follow- 

 ing da}' (our fig. 23) the shell, which has grown very much, covers a large portion of the 

 body. It already contains carbonate of lime, as shown by treatment with acids. Of 

 fig. 21:, Ilorst says, the velum now forms a very distinct part of the body, which is al- 

 most entirely covered by the shell. The surface of the velum suri-ounded by a Avreath 

 of vibratilc cilia ali-eady shows in its centre a thickening which is the beginning of the 

 cephalic shield. An oesophagus in the shape of a' funnel leads to a large, pear-shaped 



' A horny plug, the product of the cells of the shell- 

 gland, has been described in Gasteropoda and Pteropoda 

 (Lankester (H), Balfour). The typical universality of 

 the shell-gland cannot be doubted ( note, p. 297) and the lig- 

 amentous uuion of the valves of Pelecypoda occupies the 

 precise position of the shell-gland. It seems reasonable 

 to follow Lankester's suggestion, based on Pisidiuni (43) 

 and honiologize the horny ligament of Pelecypods with 

 the horny plug of embryonic Gasteropods and Pteropods. 

 To quote Lankester, " In Pisidinm as will be seen from 

 " PI. TV. figs. 38a and 39, the two calcai'eous valves do not 

 " make their earliest appearance in close contact one with 

 " the other. The central portion of the shell-gland is not 

 ' ' concerned with them ; and since it is precisely that point 

 " which in the Gasteropods cited gives rise to a cliitinous 

 "plug, may we not see in the ligament of the bivalve, 



"which occupies so precisely the required position, the 

 " homologne of tliat production." 



' The primitive slicU in {\vclas and Unio lias also been 

 observed by investigators to begin as a simple, unpaired 

 cup. 



^Lankester, in his article Slollusca (44), refers to 

 Horst's fig. 14 (our fig, 23) as the trochosphere stage. But 

 as the shell has not only considerably developed, but con- 

 sists of two valves, we think this figure is referable to 

 the class of Pelecypoda, a period later than the tro- 

 chosphere stage. Lankester's owu definition of the tro- 

 chospliere is that at this period tlie embryo has not yet 

 acquired raolluscan features and is comparable to the 

 larval forms of many Cha^topod worms and other Coelo- 

 mata, which is not the case in the figure cited. 



