PHYLOGENY OF TFIE PELECYPODA. 



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1 have not followed the early embryonic development of the oyster, and therefore, for 

 the stages considered make reference to the lyuhUcations of Professor BrooTcs (9), Dr. 

 Horst (23, 24) and Professor Huxley (26)} 



As will be seen in studying the oystei- both the soft parts and the hard, we have to deal 

 with a member of a highly peculiar and aberrant group, varying much from the charac- 

 ters commonly met with in molluscs. Many of the characters, and especially their period 

 of occurrence in the life of the individual, are well explained by the law of concentra- 

 tion and acceleration of development, which is considered in the introduction. By this 

 law a highly modified or " concentrated " type is one which acquires characteristics very 

 early in Its development, earlier than the noi-mal, so to speak, and such is found to be 

 the case in a marked degree in the oyster. 



The egg after fertilization undergoes a period of segmentation until it becomes a solid 

 mass of^clils, see Brooks' figs. 1-29 and our figs. 10-15. These stages, following our 

 classification, are considered as protembryo stages. (Nomenclature of stages, section in.) 



7 



Fig. 10. 



Fig. 11. 





FIG- ii. Fig l:i. Fig. U. 



ere 



Fig. 1.1 



Fig. 16. 



Fig. 17. 



Fig. 18. 



DEVELOPMENT OF OSTItEA EDULI.S (AFTER HOEST). 



Fig. 10. 'Rs.s. of oyster, with germinal ve.sicle and spot 

 visible witliin tlie vitellus. 



Fig. 11. Beginnina; of development; the germinal ves- 

 icle has become invisible and polar globules appear. 



Fig. 12. First stage of segmentation ; the egg is divided 

 into two spheres of unequal size (animal and vegetative). 



Fig. 13. More advanced, the egg being divided into f our 

 spherules. 



Fig. U. Stage when the egg shows one large vegeta- 

 tive sphere and several animal spheres. 



Fig. 15. More advanced stage seen from above, where 

 the vegetative sphere is divided into two spherules. 



Fig. 16. Embryo side view at tlie beginning of invagi- 

 nation (gastrular). 



Fig. 17. Embryo more developed; optical section, witli 

 invagination of the entoderm and beginning of the prc- 

 conchylian gland: ec, ectoderm; ck, entoderm; o, blasto- 

 pore ; sk, preconchylian gland. 



Fig. 18. Embryo a little older, side view : v, pediform 

 appendix; other letters as in fig. 17. 



The next stao-e in the development of the embryo, taking as a normal that which has 

 been commoidy observed by embiyologists, shonld be the formation of a mornla stage, 

 the cells ari-anging themselves in a peripheral layer around a hollow sphere. Acco.-ding 

 to the investigations of Professor Brooks and Dr. Horst, there is no true morula stage 



• In the consideration of these early stages such points 

 are selected from the writings referred to as serve to il- 

 lustrate this paper. I do not pretend in any sense to give 



an abstract or even a discussion excepting in a limited 

 sense of the early development. 



