280 Leonard W. Williams 



Summary. 



1. The primitive vertebra of Eemak or the scleromere of Bardeen is 

 not a morphological unit and, in the pig, it is not resegmented to form 

 the posterior part of the intervertebral disc and the anterior part of 

 the following vertebra. Its central part forms the annulus fibrosus and 

 the intervertebral portion of the chondrostyle from which 'arises the 

 fibro-cartilage of the intervertebral disc. Its lateral portions form 

 a large variety of structures, among which may be mentioned the ribs, 

 the neural arches (or parts of them), the costo-transverse articulations, 

 ligaments, myosepta, and perichondrium. In short, the primitive ver- 

 tebra is a mass of undifferentiated mesenchyma which is never seg- 

 mented longitudinally. 



2. The cartilage of the vertebra does not arise from a primary con- 

 densation of mesenchyma but from a secondary condensation which fol- 

 lows a loosening up of the relatively dense tissue of the scleromere. 

 In the pig, mesenchymal tissue of nearly uniform density collects around 

 the notochord of the trunk before the embryo is 7 mm. long. From this 

 time until the embryo is 9 mm. long, the intersegmental or vertebral 

 portions of this tissue become constantly looser while the midsegmental 

 or intervertebral portions probably become slightly denser. A secondary 

 condensation of the vertebral tissue takes place as the embryo grows 

 from 9 to 12 mm., and at the same time there occurs a loosening up 

 of the central part of the intervertebral disc preparatory to its secondary 

 condensation. The secondary condensation of the tissue of the verte- 

 brae and of the intervertebral discs produces precartilage. Chondrifiea- 

 tion begins when embryos are 14 to 17 mm. long, 



3. The notochord expands slightly in each vertebra at the time of the 

 formation of precartilage in all mammals studied except possibly man. 

 This vertebral expansion is usually obliterated as the vertebra chondri- 

 fies, and the vertebral portion of the notochordal sheaths and small 

 pieces of notochordal tissue occasionally or regularly retained in the 

 vertebra are destroyed before the ossification of the vertebra. Most 

 of the notochordal tissue is forced into the intervertebral disc and, 

 growing, forms the nucleus pulposus. 



4. Notochordal tissue undergoes a characteristic cytomorphosis. It 

 is primarily cellular and epithelial; later it becomes a syncytial net- 

 work with a niucin-like substance in its vacuoles; and finally it be- 

 comes cellular and closely resembles cartilage. 



