Heart, Blood Vessels, Blood, and Entodermal Derivatives 



445 



mordia for the leaflets of the valve (Patten, 

 Kramer and Barry, '46). The authors sug- 

 gest that the mouldhig effect of the spkal 

 streams of blood, as described by Bremer, 

 may act particularly on the cardiac jelly 

 and thus "set the pattern followed by the 

 endocardial cushion tissue." 



Polarity. The determination of polarity in 

 the heart rudiment has been studied for 

 several species of amphibians with some- 

 what different results. It is reported that 

 180 degree rotation of the anteroposterior 

 axis of the heart rudiment in Bombinator 

 does not interfere with normal development 

 when the operation is made in embryos with 

 a just-closed medullary tube, or earlier (Ek- 

 man, '21; Stohr, '25). A similar operation 

 at the tail-bud stage j^roduces a reversed 

 heart, indicating axial determination (Stohr, 

 '25). In Rana fusca, the anteroposterior 

 cardiac axis appears to be determined some- 

 what earlier than in Bombinator (Ekman, 

 '29). On the other hand, it is reported that 

 the heart of R. nigromaculata still lacks 

 anteroposterior axial determination when 

 it is approaching a tubular stage (Ota, '30). 

 However, since Ota reports only three posi- 

 tive cases out of 783 experiments one may 

 well question whether the hearts of the 

 positive cases developed from the rotated 

 rudiments or whether they formed by re- 

 generation from nonrotated neighboring 

 tissue. 



In Amblystoma, the anteroposterior axis 

 has been rotated 180 degrees either by a 

 single operation at tail-bud stages or by a 

 bilateral operation at stage 22, shortly after 

 closure of the neural tube (Copenhaver, 

 '26). Both types of operations produced 

 atypical, reversed hearts. W^hen one recalls 

 that the heart of a stage 25 tail-bud Am- 

 blystoma is differentiated only to a degree 

 comparable to that of a neural fold Bom- 

 binator, it appears that the anteroposterior 

 axis of the heart is determined much earlier 

 in the former species than in the latter. It 

 is suggested that the determination of the 

 anteroposterior cardiac axis shovdd be re- 

 studied to learn whether the differences de- 

 scribed for Bombinator and Amblystoma 

 are well founded and whether they exist for 

 anurans and urodeles generally. 



Determination occurs later for the dorso- 

 ventral and transverse axes than for the 

 anteroposterior axis. When the ventral half 

 of the heart rudiment of Bombinator is ro- 

 tated dorsoventrally, up to beginning tail- 

 bud stages, the parts unite to build a func- 

 tioning heart (Ekman, '29). Likewise when 



a right half of the heart rudiment of a tail- 

 bud Amblystoma is replaced with a left half, 

 thus changing the mediolateral axis of a 

 half, the parts combine to form a function- 

 ing heart (Copenhaver, '26). 



Totipotency of the Heart Rudiment. There 

 is considerable evidence that an entire 

 heart can develop from a part of its rudi- 

 ment and that each part is therefore equi- 

 potential in its early developmental stages. 

 One line of evidence comes from duplica- 

 tions occurring in nature. A most remark- 

 able case of this type was reported by 

 Verocay ('05), of a hen which had seven 

 hearts of approximately equal size. It also 

 became evident from some of the earliest 

 studies in the field of experimental embryol- 

 ogy that the heart rudiment is plastic and 

 not a fixed mosaic. It was shown in chick 

 embryos that the bilateral rudiments for 

 one heart can develop into two hearts when 

 they are prevented from uniting (Graper, 

 '07). In frogs, it was found that the rudi- 

 ments for two hearts exhibit various degrees 

 of fusion when embryos are joined along the 

 ventral region (Born, '97). 



Extensive experimental studies showing 

 totipotency in the amphibian heart rudiment 

 have been made by Ekman ('21) on embryos 

 of Bombinator. He found that a functioning 

 heart will develop after removal of a lateral 

 haK and that two hearts, each with circu- 

 lation, can develop from one rudiment split 

 lengthwise. These studies were confirmed 

 on tail-bud stage Amblystoma embryos 

 (Copenhaver, '26). They were also extended 

 to show that a functioning heart can develop 

 after removal of an anterior half rudiment 

 or from combinations of two anterior or two 

 posterior halves. On the other hand, Stohr 

 ('27) concluded from studies on Bombinator 

 and Amblystoma that the heart does not 

 fulfill the requirements of an equipoten- 

 tial system. He found that a heart develop- 

 ing from a lateral half rudiment generally 

 has an atypical shape for one or more of 

 its chambers but some of the illustrated 

 cases approximate the normal to such de- 

 gree that one may question whether they 

 argue against equipotentiality of a half rudi- 

 ment or whether they indicate merely a 

 labile determination of form. Remarkably 

 normal hearts from lateral half rudiments in 

 Amblystoma have been described by Fales 

 ('46)_. 



Evidence for equipotentiality of anterior 

 and posterior halves may be more question- 

 able than that for lateral halves. In the lat- 

 ter case, the four embryonic divisions are 



