260 Leonard W. Williams 



loosening up of the blastemal tissue which, at least in the spine of the 

 pig, always precedes the condensation that forms precartilage. 



An extraordinary multiplication of the cells of the vertebra, disc, 

 and neural and costal processes has begun in pig embryos of 11 mm. A 

 further differentiation of the discs and the vertebra accompanies this 

 new phase of growth. The tissue of the vertebrae has become precar- 

 tilage, for the nuclei stain less intensely and, although the proto])las- 

 mic network remains, it becomes attenuated and stains less readily tlian 

 elsewhere. The cells and cytoplasm of the discs, on the other hand, 

 continue to take stains as before. 



The edges of the discs are continuous Avith a similar but less dense 

 tissue which completely surrounds the vertebras and, in the median line, 

 fuses with the upper and lower edges of the perichordal septum. The 

 neural processes are possibly separated from the vertebral bodies for a 

 time by this sheet of tissue, but long before chondrification l)egins the 

 neural processes or arches are united to the vertebrae. 



'J'he notochordal cells have lost all definite arrangement and are move 

 or less vacuolated. They are flattened antcro-posteriorly and are closely 

 packed together. 



The number of cell divisions in the vertebra^ apparently reaches a 

 maximum in embryos of 12 mm. (Fig. 3), and there are 54 nuclei in 

 5,760 cubic microns. In addition to the exceptionally large number 

 of mitoses, one sees many elongated and dumb-bell-shaped nuclei, as 

 well as a few pairs of top-shaped nuclei united by their points at a large 

 acute or an obtuse angle. The three nuclei in Fig. 13, as well as a 

 number of similar nuclei, were found in a single section of one vertebra 

 (Section 894, H. E. C. No. 5). This embrj^o is well preserved, and similar 

 nuclei occur in two other 12 mm. embryos (ISTos. 6 and 518), but seem 

 to be absent from a fourth embryo (No. 7). I am inclined to believe 

 that the rapid cell division which accompanies the transformation of 

 blastemal tissue into precartilage is partly mitotic and partly amitotic. 

 Eod- and dumb-bell-shaped nuclei occur in embryos of 10 and 14 mm., 

 but they are rare and do not furnish acceptable evidence of amitosis. 



The precartilage of embryos of 12 mm. has reached its maximum 

 density. The nuclei are surrounded by small quantities of cytoplasm 

 which forms a delicate network. 



A considerable quantity of loose mesenchyma separates the vertebrae 

 and intervertebral discs from the spinal cord. Anteriorly the base 

 of the spinal cord has lost its segmental undulations, but posteriorly 



