In the vascular system at the start of the third period, 

 Baer noted the change in the heart as well as its location 

 and structure. Further development of the partition extends 

 to the stem of the aorta; at the end of the seventh day, the 

 stem widens and reveals two canals coming from the right and 

 left chambers of the heart. 



The nervous system has the following features. The hard 

 and soft brain membranes remain distinctly separated from each 

 other. The trunk part of the spinal cord has thickenings 

 where nerves exist from the extremities; however, the thicken- 

 ings which correspond to the anterior and posterior extremities 

 directly cross each other. The spinal cord nerves, regardless 

 of their insignificant thickness, could extend a fairly great 

 distance. In the brain, the predominant part is the four- 

 hillock structure, connected with the cerebellum. The deepen- 

 ing of the fissures between the divisions of the brain leads 

 to their great separation. Inside the brain, the lateral 

 ventricles appear. The funnel of the brain is well developed, 

 the brain appendage still not clearly separated from it. 

 Anterior to the funnel, there is a projection from which the 

 optical nerves go out, which do not yet form the actual 

 crossing. Baer investigated the condition of the visual 

 organs in detail. He noted a cored entrance in the optical 

 nerve, the nerve itself already compact. The retina remains 

 very thick, thicker than the cover of the brain hemisphere. 

 The vascular membrane of the eye is completely separated from 

 the still very thin hard membrane, whose continuation is the 

 cornea. The openings of the external ear lie above the mouth 

 slit. The openings of the Eustachian tubes lie close together. 

 The olfactory depressions become deep; the nostril passages go 

 to the outside from the space between the upper jaws and 

 frontal projections into the mouth cavity. 



The second stage of the third period of development is 

 described in § 10, continued throughout the eighth, ninth and 

 tenth days of incubation. At this stage, the vascular area 

 covers about three fourths the surface of the yolk. The border 

 vein disappears entirely; the veins and especially the arteries 

 of the vascular area narrow considerably. 



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