192 EMBRYOLOGY. 



vessels. They enter the sinuses at an angle, and are therefore 

 compressed by the muscular tissue of the uterus in its contracted 

 state. 



How is the vertebral column developed? 



Early in the development of an embryo there is formed beneath 

 the medullary groove in the mesoblast a thin thread of soft carti- 

 lage known as the chorda dorsalis, or notochord. This soon be- 

 comes included in a sort of fibrous sheath, and is the primary axis 

 from which the bodies of the vertebrae are developed. On either 

 side of the notochord are developed small centres which subse- 

 quently split. These are the protovertebrse. From these are de- 

 veloped the vertebrae and the heads of the ribs by the inner layer, 

 and by the outer (or posterior) layer the muscles and skin of the 

 back, the epidermis being derived from the epiblast. 



How do the protovertebrse form the vertebrae? 



It is not by direct ossification of the protovertebrae. but they 

 separate in such a way that adjacent protovertebrae each contribute 

 half to the vertebra formed. That is, two protovertebrae form parts 

 of two vertebrae, one above and the other below, and also form a 

 whole vertebra by their adjacent portions. 



How is the cranium developed ? 



By the prolongation of the epiblast over the protovertebrae to the 

 cephalic end of the embyro. Here it develops three segments, cor- 

 responding to the three primary vesicles which are the forerunners 

 of the brain. These centres of ossification are at the base of the 

 skull, the bones of the vertex being developed from membranes. 



How is the face formed in the embryo? 



At the head fold of the embryo the mesoblast does not split into 

 two parts, as below, butfolds in from the side, covered without and 

 within by the epiblast and hypoblast. These folds develop certain 

 clefts from which the face is derived, the mesoblast furnishing the 

 bone and muscle structures, and the epiblast the epidermis, while 

 the hypoblast gives the mucous membrane which lines its cavities. 



Describe the cleft which develops the face. 



Immediately below the anterior cranial vesicle there occurs on 

 either side a cleft in the lateral fold of the embryo extending to 

 the vesicle for the eye. In the space of this cleft there is devel- 



