EMBRYOS WITH TWENTY-EIGHT SEGMENTS. 283 



The secondary segments, My^ are very characteristic, and should be studied 

 with a higher power. The segment consists of an outer layer and an inner layer 

 of about equal thickness, and these two layers pass over into one another at the 

 dorsal and ventral edges of the segment. They are closely pressed against one 

 another, so that there is no space between them. The outer layer is more deeply 

 stained than the inner ; its nuclei are somewhat less distinct and are rounded in 

 form. Those of the inner layer are elongated in form, as may be easily observed 

 by raising and lowering the focus. The outer layer is quite close to the ectoderm, 

 and the inner layer rests against the large mass of mesenchymal tissue which 

 surrounds the spinal cord, notochord, and aorta. 



Section through the Anlage of the Liver (Fig. 159). — In this section the gene- 

 ral topography is similar to that of the last, so that we need describe only the new 

 structures and relations which appear. A little piece of the ventricular limb of 

 the heart with its double walls, m. ht. endo, still appears. The section is, strictly 

 speaking, beyond the venous end of the heart and passes through the sinus 

 venosus. Si. V, which is formed by the union of the omphalo-mesaraic veins 

 entering the body of the embryo from the splanchnopleure of the yolk-sac, or, in 

 other words, from the area vasculosa. In the splanchnopleure, Spl, there is a 

 thickening, x, of the mesoderm which marks the crossing of the veins from the 

 yolk-sac to the venous end of the heart. The entoderm of the embryo forms a 

 tube, Ent, which is greatly elongated in its dorso-ventral diameter. The entoderm 

 itself is quite thick, except in its median dorsal portion. From the ventral side 

 of the entodermal canal spring two small pouches or diverticula, the anlages of 

 the liver. The left diverticulum is well shown in the figure ; the right diverticulum 

 appears a few sections further on. It is especially important to note that the 

 entodermal epithelium of the hepatic diverticulum comes into immediate contact 

 with the endothelium of the blood spaces. During the later development this 

 relation is preserved, and there is a complicated intercrescence of the entodermal 

 cells constituting the liver and of the vascular endothelium.* The intercrescence 

 leads to the formation of the sinusoids, which are highly characteristic of the liver 

 and which give rise to the so-called capillaries of the hepatic lobules of the adult 

 liver. These "capillaries" are, however, always true sinusoids, and morpholog- 

 ically not capillaries at all. Owing to the junction of the veins and liver, a 

 portion of the body cavity, Cos', at the side of the pharynx is shut off from direct 

 connection with the pericardial cavity. The ridge of tissue dividing the two 

 cavities from one another is the septum transversum. If the series of sections 

 be followed through tail ward, it will be found that at this stage further back 



* A few sections anterior to this the beginning of the intercrescence is observable. 



