In the vascular system during the fourth day, the 

 following changes occur. The system of the portal vein 

 clearly separates from the system of the hollow vein. The 

 ears (auricles) of the heart increase in size, open into the 

 common venous sac, whose walls are thickened and can now be 

 called the auricle. Inside the ventricular chamber of the 

 heart, a strongly marked fold appears. 



Two primary arterial arches in the branchial arches 

 become reduced, and the third and fourth ones are enlarged. 

 In addition, the fifth arch appears. The first branchial 

 slit closes, but posteriorly to the fourth arch a new slit 

 appears, so there again appear to be three. The aorta 

 branches laterally into the intervertebral spaces. 



The extremities acquire the form of plates, situated in 

 the space between the spinal and abdominal plates. 



The changes in the nervous system during the fourth day 

 are as follows. The lateral walls of the spinal cord still 

 thicken; the upper and lower layers are divided by a groove. 

 The cerebellum is already distinctly marked. The largest 

 bladder in the cephalic brain corresponds to the four-hillock 

 parts, in which a cavity appears. The third brain cavity goes 

 down to the base of the skull and forms the cone of the brain. 

 In the frontal and sincipital regions, one can distinguish, 

 although still not completely, the lateral ventricles from each 

 other by an upward hanging fold. The lower layers of the spinal 

 cord clearly extends also into the brain at the bottom of the 

 fourth ventricle and the cavity there, forming the cone of the 

 third ventricle. The sensory nerves still have the shape of 

 hollow tubes. In the entrances into the corresponding brain 

 ventricles are seen the auditory nerve (in the fourth ventricle) , 

 the optic nerve Gin the third) , and the olfactory (in the lower 

 surface of the lateral ventricle) . 



Summarizing his observations on sensory nerve development, 

 Baer concluded, that they represent protrusions of the brain 

 into the body mass, and the sensory organs represent a modi- 

 fication of the ends of the sensory nerves. This conclusion he 

 presented most clearly, in eye development. The retina on the 

 fourth day represents a thick -walled rounded structure, connected 

 by a canal to the third brain cavity. Under the eye, there is 

 a thickening, which represents the rudiment of the upper jaw. 



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