274 



Prof. C. B. Lockwood. The 



[Dec. 15, 



carry the simile a step further, it might be said that the omphalo- 

 mesenteric veins would run round the front of the trousers opposite 

 the bend of the groins. 



An adhesion of the omphalomesenteric veins to the somatopleure, 

 at the level of the hinder end of the heart, is next described, and 

 identified with the mesocardium laterale, and is shown to be the way 

 by which the umbilical veins find a passage to the heart. Those 

 vessels develop in the somatopleure, and are by means of it brought 

 in relation with the endometrium in a manner which is described. 

 The portions of the omphalomesenteric veins which cross the ventral 

 splanchnic boundary of the coelom are held by the mesocardium 

 posterius, and by the mesocardium laterale, close to the dorsal wall 

 of the coelom, and, in consequence, as the cardiac and pleuro-peritoneal 

 portions of the coelom expand, the part bounded by the omphalo- 

 mesenteric veins remains stationary and narrow. This narrow part 

 of the coelom is named the " iter venosum," because the great veins 

 have so much to do with its formation, and, subsequently, with its 

 closure. 



The development of a septum, the septum transversum, between 

 the cardiac and pleuro-peritoneal portions of the coelom, is attributed 

 to the fixation of the omphalomesenteric veins. When, in due course, 

 the heart expands and is carried tailwards by the cranial flexure 

 and its own growth, those vessels continue to hold the ventral 

 splanchnic wall of the coelom close against the dorsal wall, and m 

 consequence it becomes retroflected behind the heart. This retro- 

 fleeted portion stretches from one mesocardium laterale to the other, 

 across the axis of the embryo ; its front surface is in contact with the 

 heart, and its hinder surface is covered with hypoblast in which the 

 liver originates ; thus a ventral diaphragm is formed between the liver 

 and the heart. 



The appearance of other somatic veins, namely, of the anterior 

 cardinals and afterwards of the posterior cardinals, is noted. The 

 former develop first and empty into the umbilical veins just as 

 they (the umbilical veins) open into the omphalomesenteric ; when 

 the posterior cardinal veins appear they join the anterior cardinals, 

 so that a portion of each of the latter nearest the heart becomes the 

 Cuvierian duct. 



Until the middle of the ninth day the embryo lies with its back to 

 the uterus. The way in which it turns its right side and afterwards 

 its venter towards the uterus is described, and also the infolding of 

 the somatopleure and splanchnopleure, and its effect upon the relations 

 of the great veins and septum transversum. 



The commencement of the umbilical veins and early formation of 

 the placenta are n3xt illustrated. The allantois of the rabbit is shown 

 to be exceedingly rudimentary, and to take no part in the formation 



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