The predecessors of the permanent kidneys, which exist 

 at a later age, are temporary organs carrying the name of 

 primary or false kidneys; in birds they are named the 

 Wolffian bodies. The primary kidneys form from the mesen- 

 teric plates; however, the method of their initial development 

 Baer considered insufficiently clear. These organs have a 

 glandular character. Along them extends a duct, the false 

 ureter, which opens in the cloaca. This duct, in Baer's 

 opinion, is formed by histological differentiation of an 

 intact structure, which is transferred later into a tube. 



In the development of primary kidneys, Baer discovered 

 a regularity other than that of the digestive glands. For 

 the latter, he described the determining role of the mucous 

 membrane, which initially forms an excretory duct, and then 

 all the branchings of the gland which only later receives 

 a network providing it with the blood vessels. The primary 

 kidneys, in his opinion, develop in a different manner, 

 particularly in the first changes in the blood vessels. 

 Already under the effect of the blood vessels, secretory 

 canals are formed. This combination of Baer's wrong ideas 

 has great historical interest, because they show his great 

 attention to the interrelations of parts of the developing 

 organism and his striving to explain, by these interrelations, 

 the processes of organ formation. 



On the sixth day of incubation, the mesenteric plates 

 form extensions; these are the foundations of the permanent 

 kidneys, in the border zone of which Miiller had seen vesicles 

 with their tubules extending inward. These small urinary 

 canals later become thin, branch, and through smaller stems 

 pour into the ureter. The genital system (divided into 

 reproductive organs and conducting passages) is formed from 

 other systems of organs which are detectable later. The 

 reproductive organs, according to Baer, are formed by the 

 expansion of the abdominal part, particularly the mesenteric 

 plates. They have the shape of elongated flat bodies, 

 located on the internal surface of the primary kidneys and 

 devoid of any defined structure. The sexual organs are 

 initially paired and identical in both sexes; then the 

 right ovary in chickens diminishes in size fairly early, 

 though in wild birds both remain the same size. The right 

 oviduct in chickens also develops less. The testicles change 



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