Ossification of the vertebrae proceeds from the anterior 

 backwards, as described in detail in the first part (I 1Z, 

 2c, 2d and llf) . The body of each vertebra has. an indepen- 

 dent point of ossification. The upper arches of the vertebrae 

 form from two halves, the opposing aggregations of dark 

 granules in both the spinal plates. 



Baer's ideas of the development of the osseous skeleton 

 and the skeleton of the extremities were perceptive. Thus, 

 the first steps of skull formation in connection with the 

 developing brain constitute the form-producing interactions 

 of parts in embryonic development. Talking about the order 

 of ossification of the skeletal elements, Baer noted that this 

 histogenetic process begins earliest in the fastest growing 

 parts of the skeleton. 



The transverse appendages of the vertebrae and ribs are 

 initially established as an entity and then separated by 

 joints. In respect to the development of the skull, Baer 

 stuck to "the vertebral theory" formulated by Oken and 

 Goethe. The skull, in Baer's opinion, is "the sum of the 

 most anterior vertebral arches." It develops like the other 

 vertebrae, only the development here is "modified by the 

 strong extension of the brain" (II 7d, p. 132 (99)). The 

 bones, of the facial part of the skull "are formed from 

 the most anterior end of the ventral plates and represent, 

 in this way, the lower arches of the cephalic vertebrae" 

 (II 7c, p. 133 (100)) . 



The extremities are formed by the expansion of the 

 layer lying above the spinal and abdominal plates and which 

 becomes observable only at the third day of incubation. The 

 fold from which the extremities form spreads upwards, down- 

 wards and externally; development upwards and downwards 

 produces the trunk part of the extremities, shoulder girdle 

 and pelvis. In Drawing 28 the original point of development 

 of the extremities is shown by the spot d". The growth 

 externally raises the crest of the fold in the form of a 

 layer, after which the foundation of the extremities is 

 divided into a stem and plate, the middle and the terminal 

 segments of the extremities. 



Concerning the development of the jaws, Baer leaned 

 towards the nature-philosophical analogy identifying the 

 jaws with the extremities. 



391 



