FUSION OF CARDIAC ANLAGES IN THE CAT 61 



ten the containing cells to endotheliiiin.'* Th(^ rosnlting vesicles 

 Bremer^ has termed angiocysts. 



In the heart the conformation of the myoepicardial mantle 

 confines the mesenchyme and angiocysts lodged within its con- 

 cavity and entails their transformation into a longitudinal 

 channel. In the embryo of 4 pairs of somites the projecting 

 ridges of the mantle still intervene between the angiocysts and 

 delay the formation of a continuous lumen. In embryos, of 7 

 to 8 somites this is nearly complete, and is so from omphalo- 

 mesenteric vein to ventral aorta in the embryo of 9 somites. 

 In all of these however, and to a less degree in still older embryos 

 there are present unannexed angiocysts and abundant mesen- 

 chyme about and especially between the endothelial tubes. 

 The mesenchyme is so gradually transformed into endothelium 

 that it is not easy to define the limit between the two stages, 

 but the endothelium is the dominant tissue by the time fusion 

 begins. The cat thus conforms to MoUier's^*^ observation of 

 the late fusion relative to condition of tissue in amniotes, the 

 fusion occurring when the heart is mesenchymatous in saurop- 

 sids, when it has become endothelial in mammals. His recog- 

 nition of a stage of solid cords antecedent to the mesenchyma- 

 tous stage, in which the accelerated fusion of anamniotes occurs, 

 is theoretically and terminologically not altogether fortunate, 

 for the undifferented cells first moving into the mesostroma 

 and subsequently gi\ing rise to a variety of products are prop- 

 erly termed mesenchyme on grounds of morphology. The term 

 does not necessarily connote any theoretical prepossessions. 

 It simply designates a position and arrangement of cells other 

 than that obtaining in the three germ-layers. In immediately 

 subsequent stages the descendants of some of these cells retain 

 this character, while other become flattened in response to the 



^ Cf. for interdermal cytodesmata, v. Szily. Anat. Anz., Bd. 24, 1903, p. 

 417; and Studnicka, Ibid., Bd. 40, 1910, p. 33. For origin of endothelium in the 

 cat. Fleischmann, A. Embryologische Untersiichungen I, 18S9; Schulte, Mem. 

 Wistar Inst. Anat. and Biol., no. 3, 1914. 

 9 Am. Jour. Anat., vol. 13, 1912. 



'"Mollier, Op cit., p. 1051. 



