362 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



form and are recognizable as embryonic blood corpuscles. Mollier 

 notes that about this stage the subintestinal strand comes into 

 extremely close relation to the yolk-cells, there being in places 

 apparently complete continuity between the two — hence the con- 

 clusion on the part of observers who did not study earlier stages 

 that the vascular strand was actually derived directly from the 

 endoderm. 



About the stage with sixteen to eighteen segments the rudiments 

 of the Duct of Cuvier and dorsal aorta become apparent, in the form 

 of cells at first scattered and later joined into strands. The aorta 

 cells anteriorly often show connexions with the sclerotomes and 



a.a.l 



L 



i l 



A. B. C D. 



Fig. 173. — Rough diagram to illustrate the form of the early rudiments of heart, vitelline 

 veins, and subintestinal vein in Triton as seen in plan. A, six mesoderm segment stage ; 

 B, fifteen segments ; C, eighteen segments ; D, twenty segments. 



a.a.l, mandibular- aortic arch ; cl, position of cloaca ; H, heart ; L, position of liver ; 

 s.i.v, subintestinal vein ; v.v, vitelline vein. 



Mollier admits that some of them may actually be derived from the 

 sclerotomes (see p. 364) though he considers that the main source of 

 origin is the upper angle of the lateral mesoderm. 



At the stage with twenty segments a network of fine channels 

 has appeared over the surface of the yolk, between it and the 

 mesoderm, foreshadowing the vitelline network of blood-vessels. 

 The subintestinal strand has become still looser in texture and 

 prolongations may be found passing from it inwards amongst the 

 yolk-cells. The heart has now attained the form of a straight tube 

 the protoplasmic strands in its interior disappearing while its super- 

 ficial cells take on an endothelial character, and are recognizable as 

 the endocardium. The splanchnic mesoderm has become closely 

 moulded round it ventrally and laterally (Fig. 172, D) forming the 

 rudiment of the myocardium and the latter begins to show con- 

 tractions causing slight movement of the fluid contents of the heart. 



