DEVELOPMENT OF THE CHICK SECO>T) PEEIOD. 307 



line of their course ; the head curves itself more and more 

 under the body, so also does the tail ; and the involucra both 

 of the head and tail again bend towards the dorsal aspect ; 

 the ocular sinuses are separated more distinctly from the an- 

 terior cerebral cell, which now lies completely underneath ; 

 the cell of the corpora quadrigemina is much enlarged ; from 

 the cell of the medulla oblongata the organ of hearing arises 

 as a vesicular eminence, and in its anterior part, a particular 

 contraction of the cerebellum is very commonly to be per- 

 ceived ; the spinal cord is now a laterally compressed tube. 

 The blood collects in the periphery of the vascular lamina 

 within a circular sinus or annular vessel, the future sinus s. 

 vena tenninalis. The heart soon parts the ventral laminae 

 from one another, like a wedge, and so forms a hernia behind 

 the point of reflection of the germinal membrane to the cra- 

 nial involucrum ; it is here that the venous trunks penetrate 

 which carry the blood from the periphery of the vascular 

 lamina to the heart. The heart itself has now become a 

 relatively narrower, and more curved or spirally twisted sac, 

 which contracts with greater vigour than heretofore. The an- 

 terior extremity of the heart divides into two crura, which 

 proceed to the cover of the future oral cavity, and run for a 

 certain way under the vertebral column, where they blend into 

 the future aorta, separate again, and give off two great trans- 

 verse branches, which lose themselves in the germinal mem- 

 brane towards the periphery of the vascular area. The blood 

 by degrees acquires a red colour. The transparent germinal 

 area continues fiddle-shaped. In the periphery the serous 

 lamina recedes still more from the other laminae of the ger- 

 minal membrane that lie under it, at the same time that it is 

 raised round the whole circumference into a fold which grows 

 with great rapidity in the beginning of the third day (fig. 338, 

 A, n, /). The whole embryo is still more bent on itself; the 

 cell of the corpora quadrigemina forms its anterior and su- 

 perior end ; the caudal end is turned in more than ever, and 

 the mucous layer following the bending, a depression is here 

 formed in the same way as we have seen one produced towards 

 the anterior extremity, at the fovea cardiaca ; the digestive 

 cavity is now a channel of considerable depth ; which, how- 

 ever, is still largely patulous towards the vitellus ; from which 

 undoubtedly it derives formative materials. 



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