HISTOGENESIS OF THE LIVER 265 



In early stages the nucleus is always in the basal portion of the 

 cell. After anastomosis they are found more centrally located. 

 In later stages they are found scattered, sometimes in the center of 

 the cell surrounded with a film of protoplasm from which threads 

 extend to the cell periphery ; often close to the lumen wall of the 

 cell and sometimes near some other point in its periphery. 



Before the tubule anlagen appear the hepatic nuclei contain 

 one, two or three large masses of chromatin which surround 

 nucleoli and are generally applied closely to the nucleus wall. 

 These chromatin masses are round or oval in shape and smooth 

 in outline except for one or two small chromatin threads which 

 maj^ extend outward from each mass. The remaining space of 

 the nucleus is filled with a clear nuclear sap through which run 

 a few delicate and faintly staining fibrils. Such nuclei are 

 characteristic of the young Acanthias embryo being found also 

 in the cells of the mesenchyma, mesothelium, walls of the medul- 

 lary canal and in the mesonephric tubules and duct. Those of 

 the mesenchyma have been fully described by McGill ('10) 

 in a study of the development of the striated muscle of the 

 oesophagus in the dogfish. The figures in Neal's ('14) recent 

 work illustrate the similar structure of the nuclei in the nervous 

 system. Figure 28 shows the structure of a number of these 

 nuclei stained with iron-hematoxylin. As the hepatic cells are 

 differentiated the chromatin masses above mentioned become 

 more irregular and there are given off from them a number of 

 chromatin strands which eventually form a coarse network. 

 The chromatin masses are reduced in size and may become 

 detached from the nuclear wall. They also may be somewhat 

 broken up and present a granular appearance. On the other 

 hand, the chromatin threads which have originated from them 

 may fuse forming secondary and generally smaller karyosomes 

 which do not surround nucleoli. These changes are shown in 

 figures 31 to 34, 38 and 39. The adult hepatic nucleus is rich in 

 chromatin which is arranged in a coarse network containing 

 several karyosomes some of which are probably the remains of 

 the original ones and some of which are the result of secondary 

 aggregation of chromatin granules from the chromatin threads. 



