238 



STUDY OF PIG EMBRYOS. 



Cl.iv 



blood from the abdominal viscera, and also delivers it to the liver. In this speci- 

 men there is quite a wide and free connection within the liver between the portal 

 and umbilical veins. In other embryos of this size such a connection does not 

 always exist. The large vena cava inferior is on the right side of the embryo, 



and passes through the liver, which thus receives 

 blood directly from the Wolffian bodies and the 

 cardinal veins. From the upper side of the liver 

 the hepatic , vein goes directly to the heart, uniting 

 with the common cardinals, which receive the 

 jugular veins from the head and the post-cardinal 

 veins from the body. The cardinal veins are now 

 very much changed. In earlier stages they ex- 

 tended from the common cardinals almost the 

 entire length of the embryo. Of this great vessel 

 there now remains connected with the common 

 cardinal only a comparatively short vessel. 



Figure 173 gives a lateral view of the pharynx, 

 in order to show the four gill-pouches, I, II, III, 

 IV, as seen from the side. The curvature of the 

 Pan.d pharynx, and its passage at its posterior end into 

 the ventral trachea and dorsal oesophagus, are 

 clearly shown. As regards the gill-pouches, the 

 first rises upward and terminates in a sharp apex; 

 the second lies nearly in the same plane as the 

 portion of the pharynx from which it arises and 

 has a prolongation toward the third pouch, and 

 the end of the prolongation bends ventralward; 

 the third is narrow as it parts from the pharynx, 

 then bends downward and forward and has a pro- 

 longation, the anlage of the thymus gland, which 

 extends toward the root of the aorta; the fourth 



Pan.v 



Tra 



Lu 



Oe 



Si 



Col 



FIG. 174. PIG EMBRYO OF 12.0 MM. 



RECONSTRUCTION FROM SERIES 5, 



TO SHOW THE ENTODERMAL CANAL, 



VIEWED FROM THE VENTRAL SIDE. 

 ( 'IP, Caecum. Cl.iv, Fourth gill-pouch. 



Col, Colon. Duod, Duodenum. 



Hep, Hepatic duct. 77, Ileum. 



La, Larynx. Lu, Lung Oe, (Esoph- also begins with a narrow stalk and has an ex- 

 panded end, one apex of which extends outward 

 (to join the cervical sinus), the other inward and 

 downward, the latter being the anlage of the post- 

 branchial body. On the dorsal side projects the 

 pedunculate hypophysis, which is developed, not 

 from the pharyngeal entoderm, but from the ectoderm of the mouth-cavity proper. 

 Figure 174 is inserted in order to give a clear idea of the entodermal canal, 

 as viewed from the ventral side. Only the posterior end of the pharynx is in- 

 cluded, and the cloaca and the allantois are omitted. The figure represents only 

 the entoderm without anv of the surrounding .mes'vlerm. 



agus. Pan.d, Dorsal pancreas. 

 Pan.v, Ventra.1 pancreas. St, 

 Stomach. Tra, Trachea. Tr.Br, 

 Tracheal bronchus. I'k.s, Yolk- 

 sac. X 14 diams. (Drawn by F. P. 

 Johnson.) 



