FRONTAL SECTIONS OF HEAD, EMBRYO OF 20 MM. 253 



portion of the smaller sinusoids are crowded with nucleated red blood-corpuscles, 

 the nuclei of which are small and deeply stained ; hence each cluster of corpuscles 

 stands out as a darker spot in the liver, for the liver cells themselves stain lightly 

 and have nuclei which, though three or four times the size of the nuclei of the 

 blood-corpuscles, yet appear relatively pale in the stained specimen. The blood- 

 corpuscles which form the clusters in the liver differ somewhat from those in the 

 active circulation, for they are smaller and show less of the characteristic hemo- 

 globin color. It is believed that the liver at this stage furnishes sites for the 

 multiplication of the blood-corpuscles, and the clusters, which are so conspicuous 

 in the organ, correspond not to blood-corpuscles in active circulation, but rather 

 to corpuscles which have found a lodging-place in the liver and are there prolifer- 

 ating. Our knowledge of the blood-forming function of the embryonic liver is 

 imperfect. Above the liver is the septum trans versum or diaphragm, Diaph' , 

 Diaph" , which is formed chiefly by mesenchyma. On the lower side of the liver 

 is another broad accumulation of mesenchyma, mes, in which is lodged the gall- 

 bladder, a small section, G. hi, of the entodermal lining of which is included. The 

 intestine. In', In" , In'", is cut several times, because at this stage the intestinal 

 canal forms several coils in the abdominal cavity below the liver and on the ven- 

 tral side of the Wolffian bodies. Below the intestines appear the curious meso- 

 thelial villi. All. vi, of the allantois (compare page 222). At this stage the villi 

 are little more than large vesicles of mesothelium, which contain in their interior 

 some coagulum and a very few mesenchymal cells, associated with which are a 

 few fibers — whether true connective-tissue fibers, or not, is undetermined. The 

 mesothelium of the villi is a very thin layer of flattened cells. 



Frontal Sections of the Head. — The three sections to be described are from a 

 special series of the head. The plane of the series was made as nearly as possible 

 transverse and at right angles to the plane of the roof of the mouth. They illus- 

 trate some important points in regard to the development of the facial region and 

 of the fore-brain. In all of the sections the differentiation of the mesoderm 

 around the brain is clearly demonstrated. The pia mater is very distinct. In 

 those parts of the sections where the brain-wall is cut obliquely, it can be dis- 



FiG. 145. — Pig, 20.0 mm. Sagittal Series 60, Section 213. 

 Ab.coe, Abdominal coelom. All.vi, Mesothelial villi of the allantois. Au.d, Right auricle, bro, Entodermal 

 bronchus. Cai^Cce'', Coelom. D.C, Duct of Cuvier. Diaph', Diaph'', Diaphragm. Du.v, Ductus ve- 

 nosus. G.bl, Gall-bladder. Gen, Genital gland. In' , In", In'", Intestine. Ki, Kidney. Li, Liver. 

 mes, Mesenchyma. msth. Pleural mesothelium. P.coe, Pleural coelom. Pleu.c, Pleural cavity. Ve, Vein 

 of liver. V. msn. Vena mesonephrica. v.s, Valvula sinistra. W.h' , W.b" , Wolffian body, x. Partition 

 separating the pleural cavity from the duct of Cuvier. Y, Partition separating the pleural cavity from the 

 ductus venosus. X 22 diams. 



