PUYLOGENY OF THE PKLECYPODA. 



2:)5 



1 liaise not followed the early emhryonic development of the oyster, and therefore, for 

 the stages considered malce reference to the 2>Mlcations of Professor Brooks (9), Dr. 

 Jlorst (23, 24) and Professor Huxley (2G)} 



Ah will be seen in stiidyiiis^ (he oyster both the soft parls and tlie hai'<l,we have to deal 

 with a meiuber of a hig-hiy peculiar and abtsiTant j^roiip, varying imieh Iroin the charac- 

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

 ol" occurrence in the lilc of the individual, are well explained by tiie law of concentra- 

 tion and acceleration of developnu^nt, which is considered in the introduction. By this 

 law a highly modified or " concentrated" type is one which acquires chai-acteristics very 

 early in its development, earlier than the normal, so to s[)eak, 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 cells, see Brooks' figs. 1-29 and our figs. 10-15. Tliese stages, following our 

 classification, are considered as pi'otembi'yo stages. (Nomenclature! of stages, section in.) 



I'lU. 11. 



tio li 



I'll;, i;; 



p^... .-i*!?!5^>, ,••■' 



Flc;. I.'). 



Flc. Hi. 



i»it-"rr~!$,- *' '.,' 



Flu. 18. 



DEVELOPMENT OF OSTUEA EDUI.IS (AFTER IIOIi.ST). 



Fig. in. Es^s: of oystor, with i;i'riiiin;il vesicle and spot 

 visil)U> witliin the vitellii.s. 



1'"IG. 11. Besiuninji of clevelopineiit ; the germinal ves- 

 icle has l)ecoiiie lnvisil)le and jxilar globules appear. 



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

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



Fici. 13. More advanced, the egg l);>ing divided into four 

 spherules. 



Fig. 14. Stage wlien tlie egg shows one large vegeta- 

 tive sphere and se\-eral animal spheres. 



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

 the vegetative sphere is divided into two splieriiles. 



Fig. k;. Embryo side view at the beginning of invagi- 

 nation (gastrular") . 



Fig. 17. Emliryo more developed; optical section, with 

 invagination of the entoderm and beginning of tlie pre- 

 concliylian gland : ec, ectoderm ; en, entoderm ; o, lilasto- 

 pore; sk, preconchylian gland. 



Fig. is. Emln-yo a little oidi'r, sidi' view : v, pedil'orm 

 appen<lix; otlier letters as in lig. 17. 



The next stage in the development of tiie embryo, taking as a normal that which has 

 been commonly ob.served by embryologists, should be the formation of a morula stage, 

 Ihc cells arranging themselves in a peripheral layer around a hollow sphere. Accoi'ding 

 to the investigations of Professor Brooks and Di'. llorst, there is no true morula stage 



' In the consideration of these early stages sneli points 

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

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



an alistract or even a discussion exceptiug in a limited 

 sense of tlic early developmi'iit. 



