grain-eating birds, appears later. Starting with the eighth 

 day, the crop appears clearly as a widening in the esophagus. 

 The liver becomes less red in color than before as a result 

 of the growth of the parenchyma and the narrowing of the blood 

 vessels. The gall bladder then appears. As a result of the 

 intensive growth and branching of the respiratory tube, the 

 lungs develop quickly. "All their dispersion,'' Baer wrote, 

 "represent on the tenth day an extensive picture" (I lOn, 

 p. 173 (112)). Next he observed the foundation of the air 

 sacs, the lower and upper larynxes. 



The kidneys become shorter, as a result of which the 

 ureters are clearly seen. Simultaneously, the Wolff's 

 bodies decrease in size. In male embryos the Wolff's bodies 

 are symmetrical, and in female the right one is somewhat 

 smaller than the left. The testicles acquire a bean-shaped 

 form, and the ovaries the shape of a triangular plate. 



The vascular system does not undergo major changes. From 

 the five pairs of arches existing from the beginning, the 

 first disappears. Then the second disappears. Later, the 

 fifth left arch disappears; the aorta still has two roots 

 which are somewhat shorter than before. 



Among the parts of the brain, one sees the most intensive 

 growth in the region of the large hemispheres, which increase 

 in size in all directions, especially in the direction of the 

 four-hillock body. Behind the latter, the cerebellum has a 

 clearly seen central lobe (the little worm) . The brain takes 

 the appearance of a fibrous structure, in the form of separated 

 thick bundles. The brain base also has crossing fibers. This 

 study of the optic nerves led Baer to the conclusion that 

 sensory nerves grow from the brain. 



The skull maintains the skin consistency. The regions of 

 the wedge -shaped and occipital bones appear more compact, and 

 so does the auditory capsule. 



The eyes, whose size Baer considered "almost monstrous," 

 occupy more than half of the whole head. By the end of the 

 tenth day, a skin edge surrounds them with a little fold, which 

 later provides the beginning for the nictitating membrane. The 

 hard membrane of the eye is very thin. The retina forms a 

 growth immersed in the hyaline body; the iris remains colorless 



333 



