of birds before hatching, on the other hand, shows that in 

 both the aorta begins with two roots. But only in birds is 

 this condition transitory (kept only to hatching) , whereas 

 in reptiles it is permanent. Baer found an interesting 

 comparison in the embryo of a lizard which had five function- 

 ing branchial vascular arches. In LACERTA AG I LIS a 

 similar condition is found before oviposition (110) . 



In the embryo of a lizard, according to Baer, blood 

 circulation can sometimes be observed under a microscope, 

 and he confirmed the presence of all mentioned vessels. 

 From his data Baer concluded that in all embryos of verte- 

 brates developing outside water there are five pairs of 

 vascular branchial arches. In addition, they are present 

 at the same time in lower forms, but in higher organized 

 forms they appear and disappear in a known sequence. 

 Amphibian larvae have four pairs of vascular arches, which 

 remain for a longer time than in higher animals. Baer 

 considered it necessary to clarify whether there was a 

 fifth (outermost) arch in an earlier period under the 

 formation of the lower jaw. The means of formation and the 

 situation of vascular arches in amphibia are the same as 

 in birds and mammals, but the distance between the anterior 

 branchial slit and oral orifice of amphibia is more signifi- 

 cant. In bony fish, throughout life four vascular arches 

 remain in the gills. In plagiostomes there are five pairs 

 of vascular branchial arches, and in cyclostomes more, but 

 the cyclostomes, Baer noticed, generally strongly deviate 

 in structure from other vertebrates. The last note is 

 interesting, because in it, on the basis of comparative 

 embryology, Baer presented a well-founded discussion of the 

 systematic situation of cyclostomes. The separate situation 

 of cyclostomes was recognized later, and they were distin- 

 guished from the class of fish in the dependent class of 

 vertebrates. 



437 



