450 



is found in tissue culture experiments (Bur- 

 rows, '11) and in some of the embryonic 

 heart experiments cited in the earlier part of 

 this discussion. The latter include the de- 

 velopment of pulsating hearts from pre- 

 sumptive heart mesoderm after explantation 

 to cultures (Ekman, '21; Stohr, '24a; Bacon, 

 '45); after transplantation to heterotopic 

 positions on the embryo (Ekman, '21; Stohr, 

 '29; Copenhaver, '26); and after transplan- 

 tation to the chorioallantoic membrane 

 (Kume, '35; Rawles, '43). Neither extrinsic 

 nor intrinsic cardiac nerves were present in 

 the experiments cited above but it may be 

 argued that the evidence does not apply to 

 the adult heart. Among the experiments sup- 

 porting a myogenic origin of heart beat in 

 the adult, the work reviewed by Haber- 

 landt ('27) has particular significance. He 

 used freezing and chemical agents for sepa- 

 rating the neural from the muscular com- 

 ponents in adult hearts. 



The embryonic heart does not become in- 

 nervated until some time after the sinus 

 venosus has become established as the pace- 

 maker. Vagus ingrowth (parasympathetic 

 innervation) generally precedes sympathetic 

 innervation, and morphological innervation 

 precedes functional control by a variable 

 period in different species. Vagus ingrowth 

 occm's: (1) in A. punctatum at Harrison's 

 stages 44-46 (about 10 to 12 days after beat 

 initiation (Copenhaver, '39b); (2) in the 

 chick at 120 hours incubation according to 

 Abel ('12); (3) in man, at the beginning 

 of the fifth week (His and Romberg, 1890). 

 In Fundulus heteroclitus embryos, Arm- 

 strong ('35) found that cardiac innervation 

 can be demonstrated by vital staining with 

 methylene blue on the eighth day, about 5 

 days after the onset of heart beat. He also 

 found that acetylcholine in small amounts 

 induces auricular diastolic arrest on the 

 eighth day whereas the same drug in large 

 amounts does not inhibit contractility pre- 

 ceding innervation. Responses cannot be 

 elicited by reflex vagus stimulation on the 

 eighth day; apparently the reflex arc is not 

 complete until about 36 hours after the vagus 

 ingrowth. Brinley ('35) obtained adrenaline 

 effects on F. heteroclitus embryos, indicating 

 the presence of a sympathetic innervation in 

 teleosts; earlier workers failed to find a 

 cardiac sympathetic iimervation in this 

 class of vertebrates. 



Functional changes correlated with vagus 

 ingrowth have been studied most completely 

 in Fvmdvdus embryos (Armstrong, '31). Sev- 



Special Vertebrate Organogenesis 



eral physiological stages can be identified 

 covering the period from the initial reflex 

 vagus response on the ninth day until an 

 adult type of response is attained on about 

 day 12-13. The type of response elicited by 

 vagus stimulation at each successive stage 

 appears to be correlated with the progressive 

 innervation of different parts of the em- 

 bryonic heart. 



The amount of cardiac control normally 

 exerted by the vagi (vagal tone) varies for 

 different species (see review by Clark, '27). 

 In most amphibians, there is very little vagal 

 tone. In this connection, it is interesting 

 to note that embryonic hearts transplanted 

 heteroplastically between A. tigrinum and 

 A. punctatum maintain their donor species 

 rhythms although they become morphologi- 

 cally and functionally innervated by the 

 host nerves (Copenhaver, '30, '39b). In these 

 cases, the basic heart rate characteristic for 

 the species is apparently myogenic. 



EMBRYOGENESIS OF BLOOD VESSELS 



The development of blood vascular endo- 

 thelium falls into two stages: (1) differ- 

 entiation in situ from mesenchymal cells; 

 (2) formation by vascular sprouts from pre- 

 viously formed vessels. An overlapping in 

 the time of the two stages was noted 

 by Sabin ('17) when she observed that 

 sprouting begins before in situ formation 

 is everywhere complete. Once the embry- 

 onic vascular system is fully established, 

 the endothelium of new vessels arises 

 only as an outgrowth from pre-existing 

 vessels. Clark and Clark ('39) have sum- 

 marized and extended earlier studies on this 

 point. 



The mesenchymal cells which form endo- 

 thelium are usually designated as angio- 

 blasts or vasoformative cells. Further studies 

 are needed to show when the earliest vaso- 

 formative cells are determined and when 

 their formation from indifferent mesenchyme 

 ceases. Lewis ('31) suggested that endo- 

 thelium and mesenchyme may be differenti- 

 ated in the chick at the time when they 

 leave the primitive streak. On the other hand, 

 Sabin ('20) described the formation of 

 new angioblasts from mesoderm throughout 

 the first two days of incubation. Since 

 Sabin's conclusions were based on cyto- 

 logical differences between angioblasts and 

 mesoderm, they do not eliminate the proba- 

 bility that some of the mesodermal cells are 

 determined as angioblasts before they can 



