606 RECORD OF CURRENT RESEARCHES RELATING TO 



In Helix and Limax the torsion docs not aj^pcar so early, and is 

 seen simultaneously in the viscera and in the shell. To explain the 

 phenomena it seems to be necessary to note the process of segmenta- 

 tion of the ovum ; but here imfortunately there is but little informa- 

 tion. The fact that organs like the kidneys, which are, as we know, 

 primarily double, are in the youngest of Gasteropod larvaD single, 

 seems to show that the asymmetry is produced prior to the commence- 

 ment of the embryonic period. 



Stomach and Liver. — Here the author directs particular attention 

 to the mode by which the organs are differentiated from the embryonic 

 digestive cavity. It is, in the first place, necessary to make a funda- 

 mental distinction between the case where the cells of the endoderm 

 possess from the first a deposit of nourishment, which comes to them 

 from the yolk — protolecithin — and that where they borrow from the 

 yolk swallowed by the larva, an amount of nourishment which may bo 

 called deuterolecithin. The former does not increase during the period 

 of development, and tends to diminish ; the latter appears during 

 that period and is rapidly absorbed. The former appears under the 

 guise of globules, which are generally small; the latter is formed 

 into compact and relatively large masses, and is not, or need not be, 

 altogether derived from the yolk. 



The Thecosomatous Pteropoda afford an example of the complete 

 absence of deuterolecithin ; while in Firoloides there is but little proto- 

 lecithin, and what there is, is rapidly absorbed. In the Gasteropoda 

 the embryonic digestive cavity is bounded in part by an endoderm in 

 which the cells are of the ordinary size, and in part by some very large 

 spherules, which are crammed with the protolecithin ; the small cells 

 of the endoderm so grow as to shut off the large spherules from their 

 connection with the digestive cavity, and these thus fall into the body- 

 cavity, where they are simply absorbed. Meanwhile the small cells be- 

 come charged with deuterolecithin ; which, after a time, disappears from 

 that part of the wall which becomes the stomach and intestine ; the 

 rest forms a pouch which develops into the liver. These facts seem to 

 show that that view is incorrect, which regards the deposition of 

 deuterolecithin as a mere episode in the development of the liver ; this 

 compound is often absent from the rudiment of the liver, and is, on 

 the other hand, often found in structures, such as the ectodermal 

 tissues of the larval Helix, which have no relation to that organ. 



Nerve-ganglia. — These structures appear to arise by somewhat 

 different processes : thus, Fol himself has observed that in the 

 Pteropoda the cerebral ganglia are formed by an invagination of the 

 ectoderm, while in the Heteropoda there is a division of the same 

 primitive layer. Bobretzky has found that these ganglia are in the 

 Prosobranchiata formed by a condensation of tissue in the mesoderm ; 

 in the aquatic Pulmonata, Fol has observed a somewhat similar pro- 

 cess, save that the mesoderm appears to have been derived directly 

 from the ectoderm ; in the terrestrial forms he has seen an ectodermic 

 invagination which was just as well marked as in the Pteropoda. 



The auditory and optic organs exhibit a very similar diversity ; 

 the Heteropoda, Prosobranchiata, and terrestrial Pulmonata have 



