Because these five pairs of arches are separated, the anterior 

 pair provides the beginning of the lower jaw and hypoglossal 

 bone, and the other four become the branchial arches. On the 

 surface of the latter the branchial platelets develop, situated 

 in two rows. 



The development of the vascular system of the fish embryo 

 can be very easily observed due to its transparency. The 

 rudiment of the heart at first is very similar to the cardiac 

 canal of birds and all other vertebrates. Its two branches 

 are united, and a curved canal is formed to the right from 

 which the vascular arches are subsequently separated. They 

 meet above, as usual in two-root aortae. In the posterior end 

 of the heart two venous branches flow. The rudiments of 

 anterior extremities have the form of small triangular 

 elevations (Figure 32, 18). 



In the moment of hatching the length of the tail consti- 

 tutes about two-fifths of the total length. The eyes are 

 pigmented, but the iris membranes have no metallic brightness, 

 which appears in the second or third day after hatching. The 

 ear is large and transparent, with the auditory drums visible 

 in it. Parts of the brain are clearly differentiated, espe- 

 cially the cerebellum, the cavity of the middle and intermediate 

 brain. The structure of the vascular system one day after 

 hatching is illustrated in Figure 32, 20. The auricle of 

 the heart is curved to the left, formed by the accretion of 

 the two venous branches. The first vascular arch goes to the 

 eye and branches in the slit of the iris membrane, while its 

 other branch is the vertebral artery. The aorta develops 

 intravertebral branches, passing into branches. The caudal 

 vein passes through the kidneys, forming ramifications in them. 

 The posterior vertebral veins (Baer equated the right posterior 

 vertebral vein to the posterior hollow vein) flow together with 

 anterior veins (from brain, ear, and occipital region in a 

 transverse branch) ; both transverse branches run around the 

 yolk sac and flow into the auricle. 



In a special work on the development of fish of 1835, 

 Baer described in detail the changes of the arterial and venous 

 systems of Cyprinus bl-iooa in the days after hatching, and 

 carried out comparisons with cyclostomes and skates (adults 

 and embryos) , and also with the embryos of birds and mammals 



433 



