LAWS OF VON BAER 521 



to the same system in other vertebrate species. This statement is particularly 

 true of the gnathostomous vertebrates (i.e., vertebrates with jaws). Conse- 

 quently, we may regard the initial generahzed stages of the embryonic or rudi- 

 mentary systems as fundamental or basic plans of the systems, morphologically 

 if not physiologically. The problem which confronts the embryo of each 

 species, once the basic conditions of the various systems have been established, 

 is to convert the generalized basic condition of each system into an adult 

 form which will enable that system to function to the advantage of the par- 

 ticular animal in the particular habitat in which it lives. The conversion of 

 the basic or primitive condition of the various systems into the adult form of 

 the systems constitutes the subject matter of Chapters 12 to 21. 



The basic conditions of the various organ systems are shown in the structure 

 of shark embryos from 10 to 20 mm. in length, frog embryos of 5 to 10 mm., 

 chick embryos from 55 to 96 hrs., pig embryos from 6 to 10 mm., crown- 

 rump length, and human embryos of lengths corresponding to 6 to 10 mm. 

 That is to say, the basic or generalized conditions of the organ systems are 

 present when primitive or generalized embryonic body form is developed. 

 It is impossible to segregate any particular length of embryo in the above- 

 mentioned series as the ideal or exact condition showing the basic condition 

 of the systems, as certain systems in one species progress faster than those 

 same systems in other species. However, a study of embryos of these desig- 

 nations serves to provide an understanding of the basic or fundamental con- 

 ditions of the various systems (figs. 257-262; also fig. 347A). 



C. Laws of von Baer 



As indicated above, the species of the vertebrate group as a whole tend to 

 follow strikingly similar (although not identical) plans of development during 

 blastulation, gastrulation, tubulation, the development of the basic plan of the 

 various systems and primitive body form. As observed in the chapters which 

 follow, the fundamental or basic plan of any particular, organ-forming system, 

 in the early embryo of one species, is comparable to the basic plan of that 

 system in other species throughout the vertebrate group. However, after these 

 basic parallelisms in early development are completed, divergences from the 

 basic plan begin to appear during the formation of the various organ systems 

 of a particular species. 



The classical statements or laws of Karl Ernst von Baer (1792-1876) de- 

 scribe a tendency which appears to be inherent in the developmental procedure 

 of any large group of animals. This developmental tendency is for generalized 

 structural features to arise first, to be remodeled later and supplanted by fea- 

 tures specific for each individual species. To interpret these laws in terms of 

 the procedure principle mentioned in Chapter 7, it may be assumed that 

 general, or common, developmental procedures first are utilized, followed by 



