354 CLASS xvi. 



its keel-shaped back turned upwards and its cavity towards the 

 yolk. The vegetative layer forms the intestinal canal, which is 

 still open below and passes into the germinal membrane. This 

 passage becomes gradually narrower and narrower, and at last 

 forms the umbilical aperture. On the second day also the forma- 

 tion of the heart begins, and the circulation of the blood is pre- 

 pared for by vessels of the germinal membrane, by which the 

 second period of the history of development is distinguished. 



The second period of development commences with the third 

 day and includes three days. The darker margin, that in the 

 beginning surrounded the transparent area (see above, p. 350), has 

 expanded into a larger circle (the area vasculosa). This area is 

 formed by the vascular layer (p. 350) which has been developed 

 between the animal and vegetative layers. In this space a vascular 

 network appears in the second half of the second day. Round the 

 area is a circular sinus or a vein (sinus terminalis, vena terminalis), 

 from which the blood by three or four principal stems returns to 

 the embryo, whilst from the embryo blood flows to the vascular 

 area by two transverse stems of which the direction consequently 

 makes a right angle with that of the venous stems 1 . The cloudy 

 circles (halones), which as early as the second day had become less 

 conspicuous, have now entirely disappeared. 



During this period the embryo acquires an envelop formed by 

 a prolongation of the animal layer which is turned up on each side 

 of it and around its two extremities and thus forms two folds which 

 gradually meet above the back and unite with each other (on the 

 fourth day). In this way the embryo is inclosed in a vesicular 

 membrane, the amnion, which is in connexion with the skin of the 

 embryo where the walls of the trunk pass into the animal layer of 

 the germinal membrane. This connecting part is the skin-navel. 

 The mucous layer, in connexion with the intestine, forms the 

 intestine-navel (ductus vitello-intestinalis) situated within the skin- 

 navel. The heart has at first the form of a bent tube, which at its 

 lower extremity receives the veins of the vascular area. The upper 

 part of this tube (afterwards changed into the bulbus aortce] gives 



1 A beautiful magnified representation of this circulation of blood has been given 

 in a large figure by PANDER in his Beitrage zur Entwiclcelungsgeschichte des Huhnchens 

 im Eie. Wiirzburg, 1817, 4to, Tab. vm. See also WAGNER Icon. Physiol. (ed. prim.) 

 Tab. iv. figs. 4, 5. 



