499 



opposite the 22d, 23d and 24th spinal segments (3d, 4th and 5th 

 lumbar); in Embryo Cr, opposite the 24th and 25th (5th lumbar 

 and 1st sacral) segments. Similar conditions exist in embryo CCXXI 

 of the Mall collection, length 12 mm, age about 5 weeks. 



During the prepelvic stage, therefore, the iliac blastema is diffe- 

 rentiated in a region more anterior with respect to the spinal segments 

 than that which the ilium later occupies. The total number of verte- 

 brae reported in each of the embryos just mentioned is as follows: 

 CLXIII, 30-31; CCXXI, 35; CIX, 35; S, 34; Cr, 34. In CCLXIII, 

 CCXXI and CIX the costal elements of the 8th to the 19th vertebrae 

 are distinctly more developed than those of the other vertebrae. The 

 thoracic vertebrae are differentiated from the others at this early 

 period. 



2. Period of Chondro fication. 



During the sixth week of embryonic development the sclero- 

 blastema of the ilium becomes united to the costal processes of the 

 sacral vertebrae and by various stages the blastemal skeleton of the 

 embryo becomes converted into a cartilagenous skeleton. From this 

 period on it is possible to distinguish distinctly the five chief regions 

 of the spinal column — cervical, thoracic, lumbar, sacral and coccygeal. 

 The following table indicates the number of vertebrae belonging to 

 each region in embryos of the second and third months of development. 



In this table are included thirty-two embryos belonging to the 

 Mall collection, nine embryos belonging to the Rosenberg collection, 

 and five belonging to the His collection. The embryos ascribed to 

 Rosenberg are the nine most definitely described of the thirteen 

 mentioned in his well known paper (Rosenberg, 1876, p. 89). Of the 

 five from the His collection, four are described by Petersen (1893) 

 and one by Hagen (1900). Want of definite description requisite for 

 the purpose prevents the inclusion in the table of the other embryos 

 utilized by Rosenberg and Petersen, as well as those studied by 

 HOLL (1882) and by Unger and Brugsch (1903). 



At the period covered by the table there is regularly present a 

 rudimentary cervical rib (Rosenberg, 1882; Leboucq, 1898). There- 

 fore, no account of its presence is given in the table. The coccygeal 

 vertebrae in young embryos can be studied to advantage only in 

 sagittal sections. In many instances, therefore, there is some doubt 

 as to the exact number of the rudimentary coccygeal vertebrae of the 

 embryos included in the table. Since the important work of Fol (1885) 



32* 



