In amphibia, during the process of development a greater 

 part of the primary vascular system remains than in mammals and 

 birds. Both roots aortae are kept; the ventricle of the heart 

 remains undivided. 



Concerning the development of extremities, Baer mentioned 

 that the anterior pair suddenly appears outwards as a result 

 of breaking the intact branchial skin fold. Later he noticed 

 the disappearance of the tail as a result of absorption of the 

 mass filling its skin. The development of the central nervous 

 system of amphibia takes place as in higher animals with 

 specified differences, which depend upon the peculiarities 

 of the brain structure. With nerves, Baer repeated the earlier 

 mistake that their appearance is caused by histological separa- 

 tion from the surrounding parts. Baer stated the development 

 of the digestive system very briefly, noticing that the diges- 

 tive canal of amphibia is formed without the untwisting of the 

 yolk sac. The development of the urinogenital system he also 

 awarded a small place. The primary kidneys, described by 

 Muller, are situated in the innermost part of the trunk; they 

 are kept until the disappearance of the tail, when the permanent 

 kidneys appear. The genital organs develop later on, while the 

 genital glands' appearance precedes the formation of the fatty 

 body. 



Turning to fish, 18 Baer noted that their development is 

 similar to the development of amphibia, because there is no 

 amnion and no allantoic sac in these two classes. Incidentally, 

 the history of development of different fish is not completely 

 equal; the differences partially depend on the greater or smaller 

 quantity of yolk ("and on the peculiarity of albumen," Baer 

 added), and also on peculiarities of organization. The ova of 

 fish are formed in the ovary; in its rudimentary layer can be 

 found yolk spheres containing embryonic vesicles and surrounded 

 by a membrane. In connection with this, Baer entered a long 

 controversy with Rathke and other authors about the question of 

 which forms earlier in the ovum — the embryonic vesicle or the 



18. The development of fish was described earlier in a separate 

 edition of a very well known work (see footnotes) . In 

 future when referring to this work, the pages of its text 

 will be shown. Illustrations are taken from it and 

 mentioned here (Figure 32) . 



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