characteristic of the large divisions of the vertebrate 

 type, appear earlier, and the features of the smaller 

 systematic groups appear later. In other words, "from the 

 more general type the more specific is formed." This same 

 regularity is true for the development of the vertebrate, 

 as well as invertebrate animals, such as the Crustacea. 



At the very early stages, both vertebrates and 

 invertebrates have what is called the primary zone. This 

 raises a question, "are not all animals basically identical 

 at the beginning of their development, and is there not for 

 all of them one general primary form," Baer confirmed that 

 such a general form actually exists, represented by the 

 vesicular stage. Birds constitute no exception because 

 the blastoderm, gradually growing over the yolk, is completed 

 by the yolk membrane, and in mammals the vesicle surrounds 

 the yolk from the first. 



Yet in the German commentary to the treatise on the 

 origin of the mammalian and human ovum,^^ Baer confirmed 

 that "the simple form of the vesicle is the general basic 

 form, by which all animals develop, not only in idea, but 

 also historically." This vesicle-shaped embryonic stage, 

 which Baer considered common to all animals, corresponds in 

 modern terminology to the blastula. It is difficult to say 

 with confidence whether this echoes Oken's idea about the 

 round- shaped original form of all the bodies of nature, or 

 if it indicates Baer's unusual perspicacity which was 

 capable of discovering this important regularity through 

 comparative anatomy. 



Bringing in his results about the similarity of early 

 embryos of different animals and about the divergent character 

 of their subsequent development, Baer formulated four 

 fundamental conclusions: 



12. "Commentar zu der Schrift: DE OVI MAMMALIUM ET 

 HOMINIS GENE SI," p. 173. 



354 



