290 RICHARD E. SCAMMON 



about 30 nun. During this period roughly one-half of the bulk 

 of the liver is made up of vascular spaces and of this half much 

 the greater part is in the form of sinusoids. Figure 14 C is a 

 section through a plane corresponding to figure 14 B, and shows 

 graphically the extent of the blood vessels at the time when they 

 form the larger part of the liver. New tubules develop in great 

 numbers during this time. Finally a period of reduction in the 

 size of the sinusoids sets in. This reduction is brought about 

 entirely by the increase in the size of tubules already formed. 

 By this increase in the parenchyma the sinusoids are reduced to 

 the 'capillary sinusoids' of Minot. This process is very notice- 

 able in embryos between 35 and 45 mm. in length. It continues 

 probably to the time of birth. Figure 14 E is of a cross section 

 of the liver of an embryo 47.3 mm. long and shows the marked 

 decrease in the size of the sinusoids at that time. 



The increase in size of the sinusoids is both actual and relative. 

 The decrease in the total area (in cross-section) of the sinusoids 

 is at first only a relative one, but later, for a short period at least, 

 it is actual as well as relative. 



A point which table 2 does not bring out is that the size of the 

 sinusoids is not determined by their position in relation to the 

 larger vessels but is dependent upon the stage of development 

 of the parenchyma with which they are interwoven. The 

 sinusoids do not form a tapering system of vessels largest near 

 the veins which receive them. This has been in a way pointed 

 out in the section upon the growth of the hepatic cylinder net- 

 work. In the lateral lobes for example, the growth of the tissue 

 is at first entirely backward and during this period sinusoids of 

 large size are generally distributed throughout the lobe (fig. 15 A). 

 After the lobes have completed the greater part of their posterior 

 growth there begins a great increase in the hepatic tissue along 

 their dorsal margins and the lobes gradually push upward on 

 either side of the intestine. With this change in the area of rapid 

 growth there also occurs a change in the sinusoids which, as is 

 shown in figure 15 B, continue to form practically half of the 

 dorsal portion of the lobes while in the ventral portions they 

 form less than 10 per cent of the total area in cross-section. 



