116 



DE VEL OPMENT. 



extremity of the embryo, and lies in the place which is afterward occupied by 

 the bodies of the vertebrae. (3) On either side of the neural canal a portion of 

 the mesoblastic layer is divided longitudinally from the rest of the mesoblast, 

 so as to form a thick column, which extends from the cephalic to the caudal 



FIG. 89. Transverse section through the dorsal region of an embryo chick, end of third day. (From Foster 

 and Balfour.) Am. Amnion. mp. Muscle-plate, cv. Cardinal vein. Ao. Dorsal aorta at the point where its 

 two roots begin to join. Ch. Notochord. Wd. Wolffian duct. Wb. Commencement of formation of Wolffian 

 body. ep. Epiblast. so. Somatopleure. hy. Hypoblast. The section passes through the place where the ali- 

 mentary canal (hy) communicates with the yolk-sac. 



extremity of the embryo on either side of the spinal canal and notochord (Fig. 

 82, A 7) ; this is the protovertebral column. From a part of it is derived the 

 vertebral column, a considerable portion at the upper and outer part being differ- 

 entiated from it and eventually forming the muscles of the back. (4) This column 

 undergoes a process of transverse segmentation and becomes converted into a 

 number of quadrilateral blocks, the protovertebral somites. The process of seg- 

 mentation commences in the cervical region and proceeds successively through 

 the other regions of the body until a number of segments are formed, which corre- 

 spond very closely to the number of the permanent vertebrae. (5) From each of 

 these protovertebral somites masses of cells are budded off towards the middle 

 line, the masses of opposite sides meeting around the notochord, which they 

 enclose, and extending dorsally around the spinal cord, which they also 

 enclose. The notochord and the spinal canal are thus surrounded by a cellular 

 mass derived from the mesoblastic layer, which constitutes the membranous matrix 

 of the vertebras. (6) The next step is the conversion of this primitive mem- 

 branous matrix into cartilage. This takes place probably about the fourth or 

 fifth week in the human embryo (Kolliker). At intervals along that portion of the 

 membranous matrix which encloses the notochord the cells become pushed apart 

 by the formation between them of a homogeneous substance and the tissue 

 becomes converted into cartilage. The regions which are thus chondrified corre- 

 spond to the intervals between the successive pairs of protovertebral somites, and 

 form the bases of the bodies of the future vertebrae, the segments of the spinal 

 column thus alternating in position with the protovertebrae. In the regions oppo- 

 site each protovertebral somite chondrification does not take place, but the mem- 

 branous matrix assumes a fibrous structure, forming the intervertebral disks. 



