92 SURVEY OF THE REPRODUCTIVE PROCESS 



observed that the multitude of bodies with the characters 

 of ova, which originally fill the egg-capsules of the Pectini- 

 branchiate Molluscs, coalesce after a time into a compara- 

 tively small number of embryonic masses, but Dr. Carpenter 

 shows very satisfactorily that the majority of the egg-like 

 bodies, in the nidimentary envelopes of these animals, are 

 not true ova, but mere masses of vitelline matter, or pos- 

 sibly, as all undergo cleavage, though with perceptible dif- 

 ference? they may be unimpregnated ova, and this their last 

 act of expiring vitality. These \dtelline spheres become 

 fused together to form a store of food-yolk, for the nutri- 

 ment of the comparatively small number of embryos which 

 are actually developed.* 



In the development of Mollusca a shell is always fonned, 

 even in those species which are afterwards naked. We find 

 also certain other organs of a provisional and temporary 

 nature, such as a contractile caudal vesicle, and anteriorly 

 two ciliated lobes, which serve as organs of locomotion, 

 when the young is discharged from the egg in an immature 

 or larva state.-|- 



Here we can hardly avoid noticing what Dr. Burnett well 

 calls " that most remarkable episode in the embryology of 

 the Mollusca — the development of certain Mollusks in 

 Holothurioidea."J The facts of the case rest on the au- 

 thority of Prof. Miiller, and are mainly these. § In certain 

 individuals of Spiapta dlgitata, one of the Holothuridse, 

 there are found from one to three sac-like bodies in the 

 general cavity of the animal, attached by their superior ex- 

 tremities to the head, and by their lower ends to the in- 



* Transac. Microse. Society, III., 17. 



t Siebold, Comp. Anat., § 229. 



X Concluding note to tlie Anatomy of the CepliaIopliora,in the transla- 

 tion of Siebold' s Compar. Anatomy. 



§ For a translation of liis paper, and some judicious comments on it, 

 see An. Nat. Hist., 2d Ser., IX., pp. 22-103. 



