334 GIl-MAN A. DREW. 



(figs. 55 and 56). The development indicates that each palp 

 appendage (fig. 56, 'pa'p.) is to be regarded as a pair of ridges 

 with an enclosed groove, developed and modified so that it 

 may be extended beyond the edges of the shell. 



Little remains to be described in this general sketch of the 

 development, further than to call attention to the formation 

 of the loops of the intestine, that are indicated in different 

 stages of development by Text-figs. M to S; to the forma- 

 tion of the cartilage pit and teeth on the valves of the shell; 

 to the formation of more gill plates and foot papillae as these 

 organs continue to grow ; to the appearance of the otocystic 

 canals about the time that the sixth pair of gill plates are 

 formed ; and to the formation of the genital organs. 



Mention should be made of a peculiar closed pouch (figs. 

 40, 48, and 63, v.), of unknown function, that lies just anterior 

 to the anterior adductor muscle. It makes its appearance in 

 embryos that are just getting the second gill lobes, and is 

 fairly conspicuous in adult animals. 



Germ Layers. 



An almost spherical embryo is formed as the result of the 

 first few cleavages (fig. 5), the cells on one side of which are 

 much larger than those on the other side. The large cells 

 extend far into the interior of the embryo, and the smaller 

 cells form a cap over the larger ones (fig. 4). 



In reaching this stage of development the embryo has 

 passed through a blastula stage, in which the cleavage cavity 

 was very small (fig. 3). As the cells become arranged in the 

 manner described, the greater part of the cleavage cavity 

 disappears. It has not been determined wliether any of it 

 remains or not. A depression appears near one side of the 

 group of larger cells at a point corresponding to the asterisk 

 in fig. 4. This depression seems to be formed by the separa- 

 tion and further division of some of the large cells, and 

 results in the formation of the gut (fig. 8, 'mg.). 



The surface cells may now be regarded as ectoderm, and 

 at least two kinds may be distinguished: small ones, which 



