505 



2) Embryo with 23 presacral vertebrae, 



7 c, 12 t, 4/5 1, 6/5 s, 



3) Embryos with 25 presacral vertebrae, . 



7 c, 13 t, 5 1, 6 s, 



^ " 113 „ 5 „ 4 J 

 The most frequent variation noted is the addition of a rib to the 

 21st vertebra. But of the four instances where this condition alone 

 was found, three belong to the embryos ot Rosenbergs's collection. In 

 only two of the thirty- two embryos belonging to Dr. Mall did the 

 costal rudiment of the 21st vertebra appear like a rib; while in several 

 instances the 12th rib was very rudimentary, and in one instance it 

 was lacking. Asymetry occurred in two instances, 4.4 Vo- 



3. Period of Ossification of the Vertebrae. 



a) Prenatal. Among the embryos in the collection of Prof. Mall, 

 changes in the cartilage of the anlages of the centres of ossification of 

 the arches and bodies of the vertebrae may be seen in embryos of 

 from 30 to 40 mm in length. Calcareous deposit in these centres 

 first appears in the cervical and first six thoracic arches and in the 

 tjo to I4 bodies of an embryo of 50 mm. In the ribs calcification 

 in the centres of ossification begins earlier (Embryos XCVI, XCV and 

 CLXXXIV). We lack as yet definite information concerning the relation 

 of development to the period of gestation. Estimate of th age of 

 similar embryos given by various authors vary considerably. In case 

 of Embryo XCVI (length 44 mm, after hardening in formalin, 5^/0 

 solution) the last menstruation occurred February 9 and the abortion 

 May 4, an interval of 84 days intervening. In case of Embryo XCV 

 (length 46 mm, after hardening in formalin, 4 **/o solution) the last 

 menstruation occurred February 23 and the abortion May 17, a period 

 of 83 days intervening. A history of Embryo CLXXXIV has not been 

 preserved. Bade, by means of the X-rays, has shown calcareous areas 

 in the ribs of an embryo of 3.4 cm, which he ascribes to the 9th week, 

 and in the ribs and some of the vertebral bodies and arches of an 

 embryo of 5.8 cm, which he ascribes to the 12th week. It is probable 

 that, as a rule, calcification in the ossification centres of the vertebrae 

 in human embryos begins in embryos about 5 cm long and three 

 months old. 



Several investigators have studied the numerical variation in the 

 vertebrae of embryos from the 4th to 9th month of development. 

 Paterson (1893) gives a table showing the variations found in thirty 



