726 NATURAL HISTOEY OP AQUATIC ANIMALS. 



the egg of Ostrea edulis is jj-o inch in diameter. Estimates based on the fignres of M. Lacaze- 

 Duthiers give dimensions of s^^o inch. These discrepancies I think are probably too great, and 

 may be due to imperfect micrometric methods. If they are real it would indicate a specific 

 difference of some importance between 0. edtilin and 0. virginica. 



The actual volume of the egg of the American Oyster would accordingly be a little more 

 than s^oooooTo cubic inch, a solid so minute that we are unable to frame any adequate con- 

 ception of its diminutiveness. Under the best conditions, as seen against a dark back-ground, 

 it is visible as a grayish-white speck ; almost an optical point. It is from this diminutive spherical 

 mass of living matter that the young Oyster is developed. The development of the embryo 

 proceeds, as far as I can make out, according to the accounts given by Davaine, Brooks, Horst, 

 and others, similarly to that of other lamellibranchs. To Hatschek' we are indebted for the most 

 secure foundation for our future embryological investigations upon this difficult group of moUusks; 

 and we must not forget to mention the very important researches of Eay Lankester (Phil. Trans., 

 1875), principally upon Pisidium. I have not been able to observe the development of the larval 

 Oyster beyond the size attained by it after the complete segmentation of the egg, the develop- 

 ment of the shell, the velum, and alimentary tract. In fact, no embryos which I have attempted 

 to rear from artificially impregnated eggs have ever lived long after the time when they began 

 to take food, which is immediately after they acquire the velum, permanent mouth, and vent, 

 and are almost or altogether covered on either side by the very symmetrical larval shells, which 

 consist of carbonate of lime laid down in a matrix of conchioline. The isolation of the conchioline 

 is readily effected by the use of acetic acid, the acid dissolving out the lime entirely. I find 

 that Brooks and Dr. Horst^ have tried a similar experiment with similar results. The latter 

 writer has also been able to watch the development of the naturally impregnated ova of Ostrea 

 edulis until a pretty advanced stage was reached. He disagrees with Brooks in his interpretation 

 of the gastrula stage, and thinks that the invagination regarded by the American investigator 

 as the blastopore must be considered to represent simply the first rudiment of the shell-gland. 

 In assuming this position, from what I have been able to gather in the course of my own investi- 

 gation of the development of the American species, I think we are bound to accept Dr. Horst's 

 determination of the homology of the shell-gland of the Oyster with that of other lamellibran- 

 chiate and cephalophorous mollusks. 



Early stages op development. — The oral invagination, according to Dr. Horst, originates 

 on the opposite or ventral side of the embryo and has no connection with the dorsal pallial 

 invagination or shell-gland. The early stages of the American and European species, like the 

 later ones, appear to present no marked differences, except that the latter appears, on the evidence 

 of Dr. Horst, Mobius, and others, to carry the ova and embryo in the mantle cavity, from which 

 the first-named author obtained his material for study, by breaking a hole through the shell near 

 the margin, so as to enable him to introduce a pipette into the pallial chamber. This method of 

 getting embryos is impossible in our native species, which has wholly different breeding habits, as 

 is proved by tlie investigations of Brooks, Winslow, Eice, and myself. How much further than 

 heretofore Messrs. Brooks and Winslow have been enabled to carry the development of our native 

 Oyster during the past season at Beaufort, North Carolina, I have not been able to learn, nor do I 

 know anything more definitely as to how much success has been attained in the artificial produc- 

 tion of Ostrea edulis from artificially-impregnated eggs at the hands of Mr. Littlewood, of England, 



'XJeber Entwickelungsgesohiclite von Teredo. Arbeiten aus dem Zool. Inst, Wien., Bd. iii. 



^BijdragetotdeKennisvandeOntwikkelingsgeschiedenisvandeOester(Osi>'eocdtt/tsL.), door Dr. E. Hoist. Tjjdschr. 

 A. Ned. Dierk. Vereen, Deel, vi, 1882. 



