THE FORMATION OF THE ORGANS THE ALIMENTARY CANAL. 203 



a later stage, the whole of the sac-like anterior part of the enteron is 

 affected by these deposits, which, however, are always greatest on 

 the ventral side. The dorsal and anterior part, with which the 

 oesophagus becomes connected, is marked off into a sac-like stonnu-h, 

 while the part that lies ventrally and more posteriorly, and which 

 contains by far the largest amount of deutolecithal constituents, 

 yields the liver. The latter, originally spherical, spon becomes lobate. 

 LEYDIG describes the gradual development which commences with a 

 few large lobes ; then, by subdivision of these, an increasing-number 

 of small ones arise, until, when the embryo is ready for birth, 

 continued division has led to the formation of numerous long 

 follicles. 



It has been observed in most cases that those parts of the entoderni 

 that are laden with nutritive substance pass over into the liver or 

 else are connected with its formation ; it appears doubtful to us 

 whether this is invariably the rule, since these parts vary greatly in 

 the position they occupy in the enteron, as will be shown later. 



The accumulation of nutritive material in the ventral entoderm is 

 still more striking in the Heteropoda than it is in Paludina. FOL, 

 in connection with the Heteropoda, speaks of a ventral nutritive sac 

 formed of immense, greatly swollen cells which is abstricted from the 

 stomach so as to become the rudiment of the liver, its glandular 

 character being soon proved by the development of several lobes. A 

 ventral nutritive sac is also found in the later stages of Limnaea ; 

 but it is expressly stated that this does not take part in the forma- 

 tion of the liver, but that the latter arises quite independently of 

 it as two small caeca which grow out at the end of the stomach 



(WOLFSON, NO. 131). 



It is impossible to ascertain the correctness of the various statements made 

 as to the manner of formation of the liver. These statements differ so greatly, 

 and in the present state of our knowledge are so difficult to compare with one 

 another, that we are justified in assuming that more careful research will 

 greatly modify them. This is all the more probable as it is evident that the 

 processes under consideration are difficult to interpret. 



In the Pteropoda also, the liver is said to arise as a finger-shaped 

 outgrowth of the ventral wall at the posterior end of the stomach, 

 near which a second outgrowth soon appears (FOL). In the Pteropod 

 larva, the nutritive material is stored up in the cells forming the 

 walls of two sac-like outgrowths of the stomach, which are at times 

 separated from the latter by stalks. These nutritive sacs, one of 

 which is usually larger than the other, differ slightly in position in 



