GENERATION AND DEVELOPMENT. 



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tissue, and at first it is perfectly solid; but, by degrees, especially after 

 its junction with a cell, or with another prolongation, or with a vessel 

 already permeable to blood, it enlarges, and a cavity then forms in its 

 interior (see Figs. 434, 435). This tissue is well calculated to illustrate 

 the various steps in the development of blood-vessels from elongating and 

 branching cells. 



In many cases a whole network of capillaries is developed from a net- 

 work of branched, embryonic connective-tissue corpuscles bv the joining 

 of thair processes, the multiplication of their nuclei, and the vacuolation 



FIG. 437. Foetal heart in successive stages of development. 1, venous extremity; 2, arterial ex- 

 tremity; 3, 3, pulmonary branches; 4, ductus arteriosus. (Dalton.) 



of the cell-substance. The vacuoles gradually coalesce till all the parti- 

 tions are broken down and the originally solid protoplasmic cell-substance 

 is, so to speak, tunneled out into a number of tubes. 



Capillaries may also be developed from cells which are originally 

 spheroidal, vacuoles form in the interior of the cells, gradually becoming 

 united by fine protoplasmic processes: by the extension of the vacuoles 

 into them, capillary tubes are gradually formed. 



Morphology. Heart. When it first appears, the heart is approximate- 

 ly tubular in form. It receives at its two posterior angles the two omphalo- 

 mesenteric veins, and gives off anteriorly the primitive aorta (Fig. 437). 



It soon, however, becomes curved somewhat in the shape of a horse- 

 shoe, with the convexity toward the right, the venous end being at the 

 same time drawn up toward the head, so that it finally lies behind and 

 somewhat to the right of the arterial. It also becomes partly divided by 

 constrictions into three cavities. 



Of these three cavities which are developed in all Vertebrata, that at 

 the venous end is the simple auricle, that at the arterial end the bulbus 

 arteriosus, and the middle one is the simple ventricle. 



These three parts of the heart contract in succession. The auricle and 

 the bulbus arteriosus at this period lie at the extremities of the horseshoe. 



