Section VII 



CHAPTER 5 



Heart, Blood Vessels, Blood, and 

 Entodermal Derivatives 



W. M. COPENHAVER 



HEART AND BLOOD VESSELS 



The cardiovascular system attains functional 

 importance relatively early in the life of an 

 embryo and consequently the prefunctional 

 phase of cardiovascular development is com- 

 pleted rapidly. Furthermore, there is gener- 

 ally a close correlation of morphogenesis and 

 functional differentiation in the later stages 

 of development. For example, as the heart 

 chambers differentiate morphologically in a 

 cephalocaudal direction (from conus toward 

 sinus venosus) there is a corresponding shift 

 in the location of the pacemaker and an ac- 

 celeration in pulsation rate. On the other 

 hand, experimental studies show a lack of 

 correlation between fvmctional and morpho- 

 logical development in the earlier stages. Pre- 

 sumptive heart mesoderm grown either as an 

 explant or as a heterotopic transplant can de- 

 velop pulsations without regard to the shape 

 of the developing heart, and the pattern of the 

 main blood vessels (aorta, gill vessels, etc.) 

 can develop to a considerable extent in em- 

 bryos experimentally deprived of circulation. 

 These are a few of the points which require 

 consideration under their respective headings 

 below. 



Localization of the Presumptive Heart Rudi- 

 ment. By using either vital dyes or surgical 

 procedures, presumptive heart material can be 

 located before there is any observable morpho- 

 logical differentiation of a heart rvidiment. In 

 amphibian embryos, the position of the pre- 

 sumptive material has been identified as early 

 as the gastrula stage by tracing the fate of 

 vitally stained regions (Vogt, '29) and by iso- 

 lation and extirpation procedures. The ap- 

 proximate locations of heart-potency material 

 in urodele amphibians at successive stages are 

 shown in Figure 157. 



The boundaries of the presumptive heart 

 material have not been studied so precisely 

 for amphibians as they have been for the 



chick by Rawles ('43). This stems from the 

 fact that the studies bearing on heart localiza- 

 tion in early amphibians have not been de- 

 vised primarily for ascertaining either the 

 exact boundaries of, or the degree of potency 

 of subdivisions of, the heart-forming area. For 

 example, isolation experiments related to lo- 

 calization have been concerned primarily 

 either with the over-all distribution of organ 

 potency areas in gastrula and neurula stages 

 (Holtfreter, '38; Mangold, '37) or with spe- 

 cific problems of heart self-differentiation 

 (Ekman, '21, '27; Stohr, '24a; Goerttler, '28; 

 Bacon, '45). 



The first visible indications of heart pri- 

 mordia in urodele amphibians are found in 

 early tail-bud stage embryos (Harrison's 

 stage 25). Figure 158 shows the amount of 

 differentiation at stage 27. In anuran am- 

 phibians, the heart primordia develop earlier 

 than in urodeles. For example, the presump- 

 tive heart material of Bombinator embryos 

 completes the migration shown for urodeles 

 in Figures 157 and 158 when the neural 

 groove has just closed (Ekman, '24). These 

 differences should be remembered when one 

 compares results of experiments on different 

 forms. 



In birds and mammals, where considerable 

 cardiac morphogenesis occurs before the lat- 

 eral rudiments unite, the heart primordia 

 are observable earlier than in amphibians. 

 In man, a cardiogenic plate appears in the 

 presomite stage (Davis, '27). In the chick, 

 the pericardial coelom appears at the one or 

 two somite stage (Rawles, '43). The localiza- 

 tion of presumptive heart material in chick 

 embryos before the primordia are observable 

 has been studied by transplanting small 

 pieces of blastoderm either to tissue culture 

 media (Olivo, '28; Rudnick, '38; Spratt, '42) 

 or to the chorioallantoic membrane (Willier 

 and Rawles, '31; Hunt, '32; Butler, '35; 

 Rawles, '43). Figure 159, after Rawles ('43), 



440 



