234 ALIMENTARY TRACT. 



the true glandular section of the organ would seem to be formed. 

 The epithelium forming the inner wall of each sack is at an early stage 

 very columnar. 



The development of the organ of Bojanus in Lamellibranchiata 

 has been studied by Lankester. He finds that it develops as a 

 paired invagination of the epiblast immediately ventral to the anus. 



Generative glands. The generative glands in Mollusca would 

 appear to be usually developed in the post-larval period, but our 

 knowledge on this subject is extremely scanty. 



In Ptei-opods Fol believes that lie has proved that the hermaphrodite 

 gland originates from two independent formations, one (the testicular) 

 epiblastic in origin, and the other (the ovarian) hypoblastic. 



These views of Fol do not appear to me nearly sufficiently substantiated 

 to be at present accepted. 



The generative glands in Cephalopoda appear to be simple 

 differentiations of the mesoblast. They are at first very closely 

 connected with the aortic heart (fig. 127, kd), but soon become com- 

 pletely separated from it. 



Alimentary tract. The formation of the archenteron, and the 

 relation of its opening to the permanent mouth and anus, has already 

 been described and needs no further elucidation. It will be con- 

 venient to treat the subject of this section under three headings 

 for each group viz. (1) the mesenteron, (2) the stomodaium, and (3) 

 the proctodseum. 



The mesenteran. In the Gasteropoda and Pteropoda the mesenteron, 

 as has already been mentioned, forms a simple sack, which may 

 however, owing to the presence of food yolk, be at first without a 

 lumen. Of this sack an anterior portion gives rise to the stomach 

 and liver, and a posterior to the intestine. This latter portion is the 

 first to be distinctly differentiated as such, and forms a'narrowish tube 

 connecting the anterior dilatation with the anus. In the meantime 

 the cells of a great part of the anterior portion of the mesenteron 

 undergo peculiar changes. They enlarge, and in each of them a deposit 

 of food material appears, which is often at any rate derived from the 

 absorption of the albumen in which the embryo floats. The cells on 

 the dorsal side, adjoining the oesophageal invagination, and the whole 

 of the cells on the ventral side do not however undergo these changes. 

 There thus arises an anterior and ventral region adjoining the oeso- 

 phagus, which becomes completely enclosed by small cells and forms 

 the true stomach. The part behind and dorsal to the stomach is 

 lined by the large nutritive cells and forms the liver. It opens into 

 the stomach at the junction of the latter with the intestine, which in 

 the later stages becomes bent somewhat forwards and to the right. 

 Still later the hepatic region becomes branched, the albuminous 

 contents of its cells are replaced by a coloured secretion, and it 

 becomes bodily converted into the liver. The stomach is usually 

 richly ciliated. 



