608 DEVELOPMENT [CHAP. XLII. 



frog are developed from a portion of yolk, which becomes separated 

 from the general mass at a very early period, and is penetrated by 

 a prolongation, posteriorly, of the vessel continuous with the 

 cavity of the heart. At first there is no appearance of a division 

 in this mass of yolk substance, which becomes separated from the 

 remainder before any trace of the alimentary canal has manifested 

 itself. Subsequently, the two organs becomes more distinctly 

 marked out. In the chick, according to the same observer, these 

 organs are formed from a cellular growth upon the surface of the 

 membrana intermedia, which is separated from the rest of this 

 membrane. At first, the two lobes of the liver are of equal size, 

 but, after a time, the right lobe preponderates, as it does in the 

 adult. 



Mr. Gray has figured the liver and pancreas of the chick. They 

 seem to be developed from two separate protrusions of the intes- 

 tinal tube, about the ninetieth hour of incubation. No vestige 

 of the spleen is to be detected at this early period. 



The following is Dr. Handfield Jones' account of the develop- 

 ment of the liver in the chick. The parenchymatous portion is 

 found to appear first ; soon afterwards, an eminence, for which 

 Dr. H. Jones proposed the name of colliculus, makes its appearance 

 on the wall of that portion of the intestine which becomes the 

 duodenum. From the latter tube pass two offsets to the liver; 

 these, however, waste, but the colliculus remains. Subsequently, 

 the cystic and hepatic ducts are developed close to the liver; they 

 extend downwards, and open at the colliculus. In fishes and 

 reptiles,- the process of development is similar. Dr. H. Jones 

 observes, that at one period the gall duct in tadpoles is lined by 

 ciliated epithelium. 



Reichert describes the formation of the columns of liver cells, 

 and their increase in number ; but he considers that the cells are 

 not invested with basement membrane. This question has, how- 

 ever, been discussed in chapter xxxiii. 



Spleen. Mr. Gray has demonstrated that the spleen arises in a 

 fold of the intestinal laminae about the 114th hour of incubation 

 in the chick; and it is probable that in the human subject its for- 

 mation takes place during the third or fourth week. It is quite 

 distinct from the pancreas from the earliest period of development. 



The first traces of the splenic vein are seen about the thirteenth 

 day of incubation ; and the first blood discs appear in the organ 

 about the eighth day. The Malpighian vesicles are not developed 

 till about the twentieth or twenty-first day, when the period of in- 



