A HITMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMIT10S. 



Felix 9 notes the same difficulty in tracing hepatic trabeculse in a slightly older 

 embryo. Careful study shows that the nuclei of the entodermal cells are slightly 

 larger than those of the mesenchyma, a point that is helpful in determining which 

 cells belong to the trabeculso. 



The Bremer embryo (4 mm.) shows a somewhat more differentiated stage in 

 the development of hepatic trabeculse. Here they form anastomosing cords 

 (Lewis) 28 . As shown by Ingalls 19 in a 4.9 mm. embryo, the trabeculse are very 

 extensive and form a large mass of anastomosing cords. Thompson 45 , however, 

 found no evidence of hepatic trabeculse. In a later note 46 he states that "the 

 transverse septum is seen before the cells of the liver bud have invaded the vessels 

 which lie in it." He shows in sections, however, a transverse septum which, like 

 the one in my embryo, is quite thick, and he apparently considers that all its cells 

 (excluding endothelial and blood cells) are mesenchymal. Judging from the 

 definiteness and size of the hepatic trabeculae of the Bremer 2 embryo, which is 

 evidently only a trifle older than Thompson's or my own, one would naturally 

 expect to find some evidence of the trabeculse in the latter two. I believe 

 that, owing to their indistinctness, it is possible that they were overlooked by 

 Thompson. 



Regarding the development of hepatic trabeculse, I believe it safe to draw 

 the following conclusions: that they arise as indefinitely outlined buds of pro- 

 liferating entodermal cells from the upper portion of the hepatic diverticulum; 

 that while they grow in the mesenchyma and anastomose with one another, their 

 cells undergo further differentiation and they become more distinctly differentiated 

 from the mesenchyma. 



I must mention briefly at this point the relation of the hepatic trabeculse to 

 the veins of the transverse septum. Janosik 20 has noted that the early hepatic 

 diverticulum in the human embryo is not related to the vitelline veins in the 

 same way as in birds. Bremer 2 states: 



"The liver cords are found growing into the mesenchyma, at a level ventral to the 

 vitelline veins; in this same mesenchyma, however, we find the branches of the vitelline 

 veins ramifying and forming plexuses, and in certain places these plexuses come into 

 intimate relation with the liver cords." 



I find with Janosik and Bremer that the hepatic diverticulum and trabeculse 

 are not in close relation to the vitelline veins. These lie dorsally and laterally to 

 the diverticulum. Somewhat vent rally and anteriorly is found the sinus venosus. 

 In the region of the hepatic trabeculse can here and there be made out minute 

 spaces which contain one or two red blood-corpuscles, apparently blood-vessels; 

 but in my specimen I have been unable to make out a definite plexus as found by 

 Bremer. 



The lower knob-like portion of the hepatic diverticulum presents no special 

 feature other than a very thick ventral wall. Brachet 1 , in a careful study of 

 the development of the liver in several different vertebrates, shows that in the 

 rabbit the hepatic diverticulum is an elongated outpocketing of the foregut, 



