26 COMMISSION OF CONSERVATION 



great numbers of questions crowding upon the investigator for solution 

 at that time, as well as that mechanical and other operations on the 

 sea or on oyster beds restrict the actual opportunities for microscopic 

 work to few occasions. During the summer of 1911 I had a few 

 limited opportunities to turn to these questions, and I now think 

 that it is possible to correlate most of the preceding observations by- 

 supposing that, in the narrowing of the mouth of the gland, a union 

 of anterior and posterior lips is effected along the dorsal line, where 

 the hinge is formed, and that the original single invagination 

 becomes separated into right and left halves, receiving and moulding 

 the glistening irregular drops of semi-fluid secretion that harden 

 into the first minute shell-valves. These thin out, broaden, and 

 take definite shape, covering more and more of the dorsal and lateral 

 surfaces of the larva, and remaining very transparent and inconspicuous 

 above an underlying refractive granular mass. The outer walls of these 

 sacs are above the little shell-valves, and constitute an epithelial layer 

 (the ectodermic cells referred to by Horst) that thins out, ruptures, and 

 continues for some time to double round the growing edges of the shell. 



Absence of movement of one part upon another at this period makes 

 it impossible to obtain clear views of the limits of some of the organs. 

 This is especially true of the mantle, which must originate out of the layer 

 of cells that form a bed for the shell, and the two doubtless keep pace in 

 growth, yet the mantle can not be distinguished until a later period, when 

 its edges become free from the body. 



By this time the archenteron has made distinct progress. The original 

 simple invagination deepened and became constricted at its mouth, 

 where growth of ectoderm w^as directed inwards, pushing the endoderm 

 before it and closing up the blastopore. This in-tucked ectoderm is the 

 primitive stomodceum, and becomes the epithelial lining of the permanent 

 mouth and oesophagus. The archenteron becomes the mesenteron or 

 stomach and intestine, the latter formed as a funnel-like outgrowth back- 

 wards from the former and meeting the ectoderm at a new point below 

 the posterior edge of the small shell to form the anus. In-tucking of 

 ectoderm also takes place here, producing a proctodseum, which 

 becomes the rectum. 



This account of the origin of the permanent mouth differs m.arlvedly from that 

 originally given by Brooks for the American oyster. He believed that the blastopore 

 is formed on the dorsal surface and becomes completely closed over by ectoderm, that a 

 new mouth is formed by invagination of the opposite side, and that the larva has to be 

 inverted for comparison with the adult. On page 23 of his 1880 work he wrote, "The 

 embryo shown in figures 32 and 36 are represented with the dorsal surface below, in 

 order to facihtate comparison with the adult, but in figure 37, and most of the following 

 figures the dorsal surface is uppermost, for more ready comparison with the adult." 

 His figures 32, 36, 37 are reproduced in my Plate V, figs. 22, 24, 25, These views rest 

 chiefly on the one point of the origin of the shell, for this would determine which is the 

 dorsal surface. 



