324 STUDY OF PIG EMBRYOS. 



mesenchymal cells, the anlage of the tunica albuginea. The interior of the organ 

 contains a number of contorted epithelioid cords of cells in which there are a cer- 

 tain number of so-called primitive ova, cells which are distinguished by their larger 

 size, rounded form, greater transparency, and spherical nuclei. The bands of cells 

 are known as the sexual cords, and they are separated from one another by loose 

 mesenchymal tissue. The cords frequently anastomose with one another. They are 

 the solid anlages of the seminiferous tubules. The question of the origin of these 

 cords has been much debated, but cannot be considered as yet settled. As to both 

 the origin and the ultimate fate of the primitive ova in the mammalian testis 

 we have only incomplete information. The cords remain solid throughout embryonic 

 life, not acquiring a central cavity until after birth. The liver is a very voluminous 

 organ permeated everywhere by sinusoidal blood-vessels which offer the greatest 

 possible variety in size. In the figure only the larger of these blood-vessels have been 

 drawn in, A large proportion 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 some- 

 what from those in the active circulation, for they are smaller, and show less of the 

 characteristic hemoglobin color. It has been demonstrated 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 proliferating. Our knowledge of the blood-forming function of the 

 embryonic liver is imperfect. Above the liver is the septum transversum or dia- 

 phragm, 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.bl, of the entodermal lining of which is included. 

 The intestine, In', In", In"', is cut several times, because at this stage the intes- 

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

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

 thelial villi, All.vi, of the allantois (compare page 253). 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 mesothe- 

 lium 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 possi- 

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

 illustrate 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 



