Heart, Blood Vessels, Blood, and Entodermal Derivatives 



453 



chiefly from the difficulty of distinguishing 

 morphologically between hemocytoblasts and 

 primitive erythioblasts. The dry imprint 

 technique indicates that the first circulating 

 cells are megaloblasts (Kirschbaum and 

 Downey, '37). An experimental analysis by 

 Block ('46) indicates that the earliest cells 

 are hemocytoblasts. He finds that when the 

 rat yolk sac is transplanted into the anterior 

 chamber of the eye, the primitive cells pro- 

 duce granulocytes as well as erythroblasts. 

 This is interpreted as evidence that the primi- 

 tive cell itself is not an erythroblast. Re- 

 gardless of whether the primitive cell is 

 an erythroblast or a hemocytoblast which 

 later develops into an erythroblast, there is 

 general agreement that the primitive eryth- 

 roblasts and erythrocytes formed in the yolk 

 sac represent a megaloblastic strain differing 

 from the definitive erythrocytes (normoblas- 

 tic series) characteristic of older embryos and 

 adults. 



Environment apparently plays a part in 

 determining whether a primitive stem cell 

 forms an erythrocyte or a leukocyte. Evi- 

 dence for this includes Dantschakoff's ('24) 

 experiments showing that when the chick 

 yolk sac is transplanted to the chorioallan- 

 toic membrane, the stem cells develop 

 mainly into granular leiikocytes whereas in 

 their normal location they develop chiefly 

 into primitive erythroblasts. Jordan and 

 Speidel ('23) suggested that the fundamental 

 stimulus for erythropoiesis is "some product 

 of cellular metabolism, probably carbon 

 dioxide." 



There is considerable evidence that a de- 

 crease in oxygen stimulates the rate of re- 

 generation of erythrocytes in adults. Studies 

 by Grant and Root ('47) on bone marrow 

 blood following hemorrhage in dogs indicate 

 that oxygen content and capacity, rather 

 than oxygen tension and saturation, affect 

 erythropoiesis. Their studies are particu- 

 larly interesting in relation to the commonly 

 held view that erythropoiesis in marrow is 

 related to a sluggish blood flow and a lo- 

 calized anoxia. They found that stagnant 

 anoxia, following hemorrhage, does not per- 

 sist any longer in marrow blood than in 

 jugular vein blood. 



A discussion of the factors necessary for 

 the maturation of the erythrocyte is beyond 

 the scope of the present review. It is particu- 

 larly interesting to note that vitamin B^o, 

 obtained from liver extract, appears to have 

 tho same effects as liver extract in pernicious 

 anemia therapy (West, '48). For a review of 

 the nutritive factors and mechanisms in the 



regeneration of erythrocytes, reference may 

 be made to Wintrobe ('50). 



ENTODERMAL DERIVATIVES 



The localization of presumptive organ 

 rudiments within the entoderm has been 

 studied by the vital dye marking method 

 (Vogt, '29; Balinsky, '47a) and by numerous 

 experiments employing the methods of extir- 

 pation and transplantation. In amphibians, 

 the different regions of the entoderm appear 

 to be capable of self-differentiation in the 

 gastrula stage as evidenced by the behavior 

 of transplants in the coelomic cavity and by 

 explants in modified Ringer's solution (Holt- 

 freter, '25, '38). On the other hand, evidence 

 for alteration of the prospective fate of 

 amphibian entoderm is seen in the occasional 

 development of muscle from entodermal cells 

 transplanted from gastrulae into the eye 

 chamber of older larvae (Kusche, '29). His- 

 tological regulation in amphibian entoderm 

 has been described for neurula stages also 

 (Balinsky, '38). However, studies on the tree 

 frog by Kemp ('46) are more in conformity 

 with Holtfreter's views and indicate that 

 the entoderm in the neurula is unable to 

 regulate histologically although it is capable 

 of considerable morphological regulation. 



Extending the studies on amphibian ento- 

 derm to earlier stages than those used by 

 Holtfreter, Nicholas ('48) found that the 

 entoderm already possesses some determina- 

 tion in the late blastula stage. When the 

 entire cover of an early gastrula is removed, 

 the entoderm differentiates stomodeal and 

 proctodeal pits. The factors for the formation 

 of these two structures are apparently in- 

 trinsic to the yolk entoderm at an early 

 stage. 



An understanding of the formation of ento- 

 dermal structures in the chick has been 

 aided particularly by the method of chorio- 

 allantoic grafting. Early blastoderms can 

 be split into two layers: epiblast and ento- 

 derm. Grafts have been made of entire layers, 

 of parts of layers, and of small areas of total 

 thickness blastoderms. The experiments indi- 

 cate that: (1) the gut-forming potency is 

 limited to the epiblast in prestreak and early 

 streak blastoderms (Hunt, '37a); (2) the 

 definitive entoderm does not become deter- 

 mined until just prior to the head-process 

 stage (Himt, '37a; Rudnick and Rawles, 

 '37); (3) for a time there is an overlapping 

 of gut potentialities in the epiblast and ento- 

 dermal layers; (4) there is a progressive 

 localization of primordia within the ento- 



