162 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



dorsal side, which he considers the external opening of the gastrula — 

 the blastopore. If, however, we compare his Fig. 32 {op. cit.) with my 

 figures 5, G, and 8, then I believe that we may infer with great prob- 

 ability that the structure regarded as the blastopore by Brooks is noth- 

 ing more than the external opening of the shell gland. This view is 

 further sustained by the fact that he observed that at a later period 

 the shell began to develop at this point, regarded by him as the open- 

 ing of the blastopore. Such a mode of development of the shell of 

 lamellibranchs has hitherto been observed only by Eabl* in Unio, and 

 is so entirely opposed to the observations which have been made on 

 the development of other lamellibranchs that, as has been capably 

 observed, the matter should be more closely reinvestigated. 



Returning to the embryo represented in Fig. 6, we see that the ento- 

 •dermal field or area, which in an earlier stage (Fig. 4) presents as yet 

 little more than a slight depression, has now acquired the form of a 

 deep invagination with a tubular cavity, the true gastrula form (pro- 

 togaster) ; behind the mouth of the gastrular opening lie a pair of large 

 cells, which may apparently be regarded as the first mesodermal cells, 

 although their mode of origin as well as their further development I 

 have failed to discover. In the embryo of the following day (Fig. 8) 

 one already encounters mesoderm cells on the dorsal side of the rudi- 

 mentary intestine. The ventral portion of the embryo which lies below 

 the mouth now begins to be pushed out, so that a foot-like prominence 

 is developed, whereby the embryo assumes some likeness to a young 

 gastropod. The blastopore is still very distinctly visible, and has a 

 peculiar triangular form, as seen from the anterior end, as in Fig. 7. 

 As far as I have been able to make out, the blastopore does not close, 

 but is transformed directly into the permanent mouth. For even in 

 those forms in which the blastopore closes, the oesophagus as well as 

 the permanent mouth is formed by an invagination of the ectoderm 

 and also in those in which the blastopore does not close, the ectoderm 

 cells have a share in building up the anterior portion of the alimentary 

 canal. 



During the further growth of the embryo, great internal as well as 

 externa] changes take place; which is true in the first place in regard 

 to the shell gland, which gradually loses its original character of a 

 glandular invagination ; its walls are reflected outwards, so that it again 

 becomes merely a shallow depression, with a thickened floor of long 

 conical cells (Fig. 9, sli). A cuticular membrane, s, the product of the 

 secretion of these cells, represents the primitive foundation of the shell, 

 and upon this point, in the full grown animal, rests the hinge. Accord- 

 ingly, the representations of Davainc, who remarks, a Un trait transpa- 

 rent * * * * c'est le premier indicode la charmere," are fully borne 

 out. The bivalve shell of the oyster is thus plainly seen to develop from 



* Ueber die Eutwiokelungsgeschichte der Malcrmuschel. (Jen. Zeitschrift, XL, 

 187G.) 



