the brain, while the other nerves develop by means of 

 histological differentiation in other primary organs. 



From his comparison of the three types of differentia- 

 tion Baer generated the following aphorisms. "Primary, 

 morphological and histological differentiations repeat the 

 same distinctions, the first above the others, the second 

 behind the others, and the third in the others" (II 6D, 

 p. 126 (94)). Therefore, these distinctions are not 

 absolute, but only relative, because the distinctions, 

 which are essential in the primary organs, are repeated 

 as subordinated distinctions in individual parts of the 

 body. C102) 



The divisions concluding the description of bird 

 development (§ 7) considers the formation of individual 

 systems and organs. Baer first addressed the histological 

 differentiation of the skeletal parts of the bony or osseous 

 system. According to his description, all the bones are 

 composed of cartilage, which comes from closely arranged 

 small dark globules. The mass of these globules becomes 

 light and forms a soft cartilaginous material; the periphery 

 of the cartilage becomes a cartilaginous membrane, and the 

 middle becomes a firmer cartilage. The cartilaginous parts 

 at first remain formless and only later develop defined 

 features, acquire appendages, and so on. In other words, 

 for the skeletal elements, morphological differentiation is 

 preceded by histological. Ossification in individual 

 cartilage proceeds from the middle to the surface, frequently 

 beginning in several spots at the same. time. The joints 

 appear simultaneously with the cartilage by similar histo- 

 logical differentiation, as distinctly observed by Tredern 

 on the fingers of the anterior and posterior extremities. 

 From the parts of the body system, the axial skeleton is 

 established earliest. Along the embryonic axis, dark 

 granules form a thin string, the vertebral cord. Baer 

 reminded his readers that in De ovi he gave this formation 

 the name spinal cord and later concluded that it should have 

 been named the vertebral cord. It must be noted that in 

 embryological literature after Baer his first choice gained 

 distribution. 



By means of histological differentiation, the cord is 

 separated from its lighter membrane, the cord sheath. 



390 



