x FOWLTHIRD DAY 537 



of the vascular system should be studied, the arteries are seen to be 

 more deeply coloured and more conspicuous than the veins. The 

 two vitellim- u-iiis by their fusion form the hind end of the tubular 

 heart and on traein- iliis forwards a somewhat Y-shaped vessel is 

 seen opening into it laterally. The stalk of the Y which is very 

 short, though showing considerable variability within its limits, is 

 ill.- right duct of < Juvier (Fig. 241, A, d.C). The branches of the Y 

 are the cardinal veins. Of these the posterior (p.c.v), coming from 

 the region of the kidneys, is only visible for a short distance, bciiiLi 

 soon hidden as it is traced backwards beneath the myotomes. The 

 anterior cardinal vein (a.c.v) on the other hand can be traced 

 forwards for a long distance into the head from which it drains the 

 blood back towards the heart. It will be noted that here in the 

 embryonic Bird we find exactly the same arrangement of main 

 veins duct of Cuvier, anterior cardinal and posterior cardinal as 



som. 



KM,. 'j:37A. --Transverse sections through third-day Fowl embryo. (Partly based on figures 

 by Duval.) A, Through the hinder part of the trunk region. 



A, dorsal aortue ; am, ainniotic folds ; end, endoderm ; my, myotome ; s.c, spinal cord ; 

 .-//(, somatopleure ; s/<f, splanclmopleure ; splc, splanchnocoele. 



is characteristic of the adult condition of lowly organized fish-like 

 Vertebrates. 



For the study of such details of structure as cannot be made out 

 in the whole embryo the most useful sections are series cut trans- 

 versely to the long axis of the trunk region. These should be 

 supplemented by series parallel to the sagittal plane in the head 

 region. 



It is well to commence the study of the transverse sections 

 with one through the hinder trunk region, about the level of the 

 vitelline arteries. Such a section is depicted in Fig. 237A. 



In comparing this section with a corresponding section through 

 the second-day chick (Fig. 234A) the same general features will l>e 

 recognized the differences being mainly differences in detail. The 

 most conspicuous of these is caused by the development of the 

 amniotic fold of the somatopleure which rises up on each side, 

 urchin- tnw.mls the median plane over the dorsal side of the embryo 

 (am). Traced forwards through the series the amniotic folds of the 

 two sides are seen to meet and undergo fusion so as to give rise 



