902 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [12] 



analogous development of tlie shell in the laiuellibrancliiates has, so fur, 

 only been discovered by Mr. EabP" in the U}iio, and is so utterly differ- 

 ent from the observations relative to the development of other mollusks 

 that competent authorities have already dejnauded a full verification of 

 the facts. 



To return to the embryo represented in Fig. 10, we find that the en- 

 todermal area, which in a previous stage showed only a slight depres- 

 sion, has become a deep invagination with a tubular cavity, a genuine 

 protogastrea. Behind the mouth there are found a couple of large cells, 

 which may probably be considered as the first cells of the mesoderm, 

 although the manner in which they originate, and the subsequent de- 

 velopment of the mesoderm, have escaped my observation. However 

 this may be, one finds in the embryo on the following day (Fig. 12) mes- 

 odermic cells on the upper portion of the intestinal mass. The portion 

 on the ventral side, located below the mouth, now begins to protrude 

 very strongly, so as to form a sort of foot which causes the embryo to 

 resemble a yonng gasteropod. The blastoijore continues to be very 

 distinct, and presents a somewhat triangular form. As far as I could 

 ascertain, it did not disappear, but remains and is transformed into the 

 mouth or rather into the cardia. For in the same way as in embryos 

 where the blastopore closes, the esophagus and the mouth are formed 

 by an invagination of the ectoderm ; thus in embryos where the blas- 

 topore remains, there are ectodermic cells which take a part in the for- 

 mation of the upper portion of the intestinal canal. 



Great changes, both internal and external, take place during the 

 further growth of the embryo ; the precouchylian gland gradually loses 

 its primitive character of a glandular invagination, and it reassumes 

 its primitive character and forms a thickening of the ectoderm, com- 

 130sed of long conical cells (Fig. 13, si-). A thin cuticular membrane 

 (s), produced by secretion from these cells, is tbe first indication of the 

 shell. As at this point the hinge is found in the grown oyster, the 

 description of Davaine, " a transparent streak * * * this is the first 

 indication of the hinge,'' is perfectly correct. The bivalve shell of the 

 oyster, therefore, originates in a single piece, and contrary to the obser- 

 vations of De Lacaze-Duthiers, according to which the two valves are 

 produced " by two thickenings of the epiblast," which are supposed 

 afterwards to unite and form the hinge. Brooks also states that the 

 shell of the American oyster is from the very beginning comjiosed of 

 two valves, which develop from a small, irregular, and transparent organ 

 located on each side of the dorsal furrow (his blastopore). 



If we call to mind the fact, already referred to, that the true nature 

 of this furrow and the real blastopore have escaped the observation of 

 the American naturalist, we are certainly justified in doubting the cor- 

 rectness of his observation. 



'^Ueber die Entwicklungsgeschiohte der Malermuschel, in Jeu. Zeitschr., is, 1875. 



