TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO 105 



continuation of the portal vein in the liver. This common trunk drains 

 into the ductus venosus. 



The anterior cardinal veins (Figs. 103 and 104) are formed to drain 

 the plexus of veins on each side of the head. These vessels extend caudad 

 and lie lateral to the ventral portion of the myelencephalon. Each 

 anterior cardinal vein receives branches from the sides of the myelen- 

 cephalon, then curves ventrad, is joined by the lingua-facial vein from 

 the branchial arches and at once unites with the posterior cardinal of 

 the same side to form the common cardinal vein. This, as we have seen, 

 opens into the sinus venosus. 



The posterior cardinal veins develop on each side in the mesonephric 

 ridge, dorso-lateral to the mesonephros (Figs. 103, 104 and 112). Run- 

 ning cephalad, they join the anterior cardinal veins. When the mesoneph- 

 roi become prominent, as at this stage, the middle third of each posterior 

 cardinal is broken up into sinusoids. Sinusoids extend from the posterior 

 cardinal vein ventrally around both the lateral and medial surfaces of the 

 mesonephros. The median sinusoids anastomose longitudinally and 

 form the subcardinal veins, right and left. The subcardinals lie along the 

 median surfaces of the mesonephroi, more ventrad than the posterior 

 cardinals with which they are connected at either end. There is a trans- 

 verse capillary anastomosis between them, cranial and caudal to the 

 permanent trunk of the vitelline artery (Fig. 103). The right subcardinal 

 is connected with the liver sinusoids through a small vein which develops 

 in the mesenchyme of the plica venae cavae (caval mesentery) located 

 to the right of the mesentery (Fig. 112). This vein now carries blood 

 direct to the heart from the right posterior cardinal and right subcardinal, 

 by way of the liver sinusoids and the right vitelline trunk (common 

 hepatic vein). Eventually the unpaired inferior vena cava forms in the 

 course of these four vessels. (For the development of the inferior vena 

 cava see Chapter IX.) 



TRANSVERSE SECTIONS OF A Six MM. PIG EMBRYO 



Having acquainted himself with the anatomy of the embryo from 

 the study of dissections and reconstructions, the student should examine 

 serial sections cut in the plane indicated by guide lines on Fig. 105. 

 Refer back to the external structure of the embryo (Fig. 93), to the lateral 

 dissection of the organs (Fig. 95), and having determined the exact plane 

 of each section, interpret the structures seen by comparing with Fig. 105. 

 The various structures may be recognized by referring to the figures of 

 sections in the text, and they should be traced section by section through 

 the series as carefully as time will allow. Remember that the sections 

 of pig embryos figured here are drawn from the cephalic surface, so that 

 the right side of the section is the left side of the embryo. 



