350 TEXT-BOOK OF EMBRYOLOGY. 



right and left lobes. The right becomes the larger. The right umbilical vein 

 loses its connection with the liver (p. 264). After birth the left, which lies be- 

 tween the right and left lobes, degenerates into the round ligament of the liver. 

 The other lobes arise secondarily as outgrowths from the right primary dorso- 

 lateral lobe, the caudate (lobe of Spigelius) from its inner (medial) surface, 

 the quadrate from its dorsal surface. 



The liver as a whole grows rapidly and by the second month is relatively 

 large. During the third month it fills the greater part of the abdominal cavity. 

 After the fifth month it grows less rapidly and the other intraabdominal organs 

 overtake it, so to speak, although at birth it forms one-eighteenth the total 

 weight of the body. After birth it actually diminishes in size. The right lobe 

 is from the beginning larger than the left, and after birth the predominance 

 increases. 



Histogenesis of the Liver. — The hepatic part (pars hepatica) of the 

 liver anlage is derived from the entodermal lining of the gut and constitutes a 

 mass of cells with no lumen. From this mass, solid bud-like evaginations grow 

 into the mesentery, break up the omphalomesenteric veins into smaller channels 

 and form trabecular, or hepatic cylinders (p. 348). The latter anastomose 

 freely with one another and are composed of polyhedral, darkly staining cells 

 with vesicular nuclei (Fig. 315, A). Lumina begin to appear in the cylinders 

 about the fourth week as small cavities which communicate with the cavity of 

 the gut. 



The hepatic cylinders are the forerunners of the hepatic cords or cords of 

 liver cells. There are two views as to the manner of transformation. The 

 older view is that the cylinders gradually become stretched, the number of cells 

 in cross-section becoming less until it is reduced to two. Between these two 

 lies the lumen of the cord or the so-called "bile capillary" (Fig. 315, B). The 

 other view is that branches from the sinusoids grow into the cylinders and sub- 

 divide them into hepatic cords. 



As stated above^ the hepatic cylinders are at first composed of darkly stain- 

 ing, polyhedral cells with vesicular nuclei. These are the liver cells proper. 

 Later other small spherical cells, with dense nuclei, appear and during the 

 fourth month become very numerous (Fig. 315, A). From this time on, they 

 grow less in number and at birth have practically disappeared. Earlier investi- 

 gators considered them as developing liver cells. Further study on the develop- 

 ment of the blood, however, has led others to consider them as erythroblasts 

 (p. 271). Since they are inside of the hepatic cylinders, they either wander in 

 from the intertrabecular blood vessels or lie in intratrabecular vessels. The 

 latter supposition accords with the view that the cylinders are broken up into 

 hepatic cords by the ingrowth of branches from the sinusoids. 



The development of the lobules of the liver, producing the peculiar relations 



