features in ontogenesis. If similarity is recognized 

 between the developmental stage and the adult stages of 

 lower organized animals, then this, Baer considered, occurs 

 most frequently because the given adult form is closer to 

 the original type. As a result, arrested development of 

 the more highly organized animal will certainly draw these 

 forms closer to each other. 



The second corollary, called "Application of the 

 present opinion to the determination of individual organs 

 in different animal forms," concerns rational nomenclature 

 for comparative anatomy, which can be constructed only with 

 consideration of the history of development. Thus, the 

 series of abdominal nerve knots of articulated animals cannot 

 be called a spinal cord and compared to the nodes of the 

 vertebrate sympathetic nervous system. The anterior pair of 

 nodes in articulated animals cannot be called the cephalic 

 brain. The supporters of such suggestions refer to these 

 nodes, but Baer showed, illustrating his idea by a simple 

 graphical scheme, that these nodes lie not above but 

 anteriorly to the gullet. Thus the nerve ring near the 

 gullet represents a secondary formation, dependent on the 

 breakdown of the mouth at the abdominal surface of the body. 

 Equally, one cannot compare the respiratory tubule of insects 

 with the respiratory tube of vertebrates, because the latter 

 develop from the mucodermal tubes, and the former represent 

 a result of the protrusion of the external covers. At the 

 same time, in a number of types there are organs of identical 

 purpose and origin. Thus, "the digestive canals in all 

 animals are formed from the membrane facing the yolk." 

 This example and a number of others, Baer put as basic to 

 the study of analogous and homologous organs; the demarcation 

 of these understandings are attributed usually to a later 

 period of the history of biology and connected with the name 

 Richard Owen. 



In the third corollary, "Application to the knowledge 

 of animal affinities," Baer again returned to the idea of 

 the linear or stair-like succession from the cephalopods or 

 Crustacea to fish, from the echinodermata to the molluscs, 

 the impossibility of knowing which stands higher — the 

 articulated animals or the molluscs — and so on. If the 

 current idea about the ladder of animals were renounced, 



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