298 



ROBERT TRACY JACKSON ON THE 



Dr. Ilorst says that great changes take phu-e durhig farther growth of tlie embiyo. 

 The preconch} liaii gland gi-adually loses its primitive chai-actcr of a glandulai- invagi- 

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

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

 at this position in the adult the hinge is found.' ITorst maintains, therefore, that the 

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

 Brooks in Anodon and Ilatschek in Tei'edo.'^ 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 lias formed a spacious cavity in the base of which -is a diverticu- 

 lum, now closed, but which later communicates with the ectoderm and forms the poste- 

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

 resented b}' the trochosphere stage befoi-e the development of the shell-gland, Pi. xxm, 

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

 inates early, closely following the gastrula invagination and before the formation of a 

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

 stage as skipped in the oyster by the ovei'lai)ping of stages due to extreme concenti'ation 

 and acceleration of development.'^ Fig. 22, on the other hand, is the nearest approach to 

 the ideal typembryo stage that we have in the developing oyster (definition of typem- 

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

 ectoderm b}' 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 comparatively high characters in the young oyster, 

 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 highly concentrated development of this type. 



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

 ing day (our fig. 23) the shell, which has grown very much, covei's a large portion of the 

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

 fig. 24, Horst 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 surrounded by a wreath 

 of vibratile cilia already 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 (44), Balfour). The typical universality of 

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

 amentous union of the valves of Pelecypoda occupies the 

 precise position of the shell-gland. It seems reasonable 

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

 and homologize the horny ligament of Pelccypods with 

 the horny plug of embryonic Gasteropods and Pteropods. 

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

 " PI. IV, flgs. 38a and 39, the two calcareous 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 chitinous 

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



"which occupies so precisely the required position, the 

 " homologue of that production." 



''The primitive shell in Cyclas and Unio has also been 

 observed by Investigators to begin as a simple, unpaired 

 cup. 



'Lankester, in his article Mollusca (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 own definition of the tro- 

 chosphere is that at this period the embryo has not yet 

 acquired moUuscau features and is comparable to the 

 larval forms of many Chfetopod worms and other Coelo- 

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



