The embryo was very small, although it was well formed. Its 

 head constituted about two-fifths of the total length. The 

 eyes were surrounded by rudiments of eyelids. In the cavity 

 of the pharynx appeared the orifices of the Eustachian tube. 

 The structure of the upper jaw of the embryo shows a similarity 

 to amphibia and tortoises, and the nasal passages of the 

 posterior orifices opened in the most anterior part of the 

 oral cavity. Behind the lower jaw, a pair of branchial slits 

 appeared. Baer considered this a second pair, which remained 

 in all animals for a longer time than the other pairs. In 

 the anterior extremities the shoulder and elbow joints were 

 very clearly seen. The wrist was not separated by a joint 

 from the forearm; it had the form of a rounded blade with 

 five fingers of nearly equal length, except for the noticeably 

 shorter large finger. The spaces between the fingers were 

 narrow, with a semitransparent connecting mass very similar 

 to the swimming membrane. In the posterior extremities the 

 same could be differentiated, but with the fingers less clear. 

 From this Baer could conclude 



that the fingers of hands and feet formed not as 

 a result of a simple thickening of the forming 

 mass into separated arms, but because the existing 

 mass collected in these arms, and because the 

 thickness of space between the fingers was less 

 than the thickness of undifferentiated plates. 

 But the most interesting was the similarity of 

 the extremities. The general agreement presented 

 by them was at once striking to the eye. (p. 51) 



The liver occupied nearly half of the abdominal cavity. The 

 stomach was well isolated from the intestine, with the form 

 of a blind sac, displaced to the left from the middle plane 

 of the body. Behind the great curvature of the stomach 

 was the spleen. The intestine near the stomach (the duodenum) 

 was slightly curved, which led directly to the place of 

 fixation of the umbilical cord, where the loop forms. The 

 caecum was short, but completely and clearly formed. The 

 left ventricles of the heart in this stage were noticeably 

 larger than the right. The roots of the aorta were only 

 slightly separated from each other. The larynx was barely 

 visible; the rings of trachea were still undifferentiated. 

 The ducts of the primary kidneys opened in the nearly cylin- 

 drical allantoic sac. The rudiments of the primary kidneys 



448 



