1622 A MANUAL OF ANATOMY 



itself only persisting as the central pulp of each intervertebral disc; 

 and (3) the formation of the mesodermic somites from the paraxial 

 mesoderm, these somites being arranged in two longitudinal rows, 

 right and left, one on either side of the neural canal and notochord. 

 The lateral plate of mesoderm on either side has been divided into 

 two lamellae — ^namely, somatic and splanchnic, this being effected 

 by the formation, within each lateral plate, of vacuoles, which 

 coalesce and so give rise to a cleft -like space. Within this space 

 fluid accumulates and a cavity is formed, called the ccelom, primitive- 

 hody-cavity , or pleuro-peritoneal cavity. The somatic mesoderm joins 

 the ectoderm, and forms the somatopleure, whilst the splanchnic 

 mesoderm joins the entoderm, and forms the splanchnopleure. The 

 cavity of the ccelom is therefore bounded on the one hand by 

 somatopleure, and on the other by splanchnopleure. 



The cavity of the blastodermic vesicle, which is situated on the 

 ventral aspect of the notochord, is single at this stage, and is bounded 

 by splanchnopleure. This cavity represents the primitive intes- 

 tinal cavity. 



Separation of the Embryo. — ^The primitive embryo, developed 

 from the embryonic area of the blastodermic vesicle, becomes 

 separated from the extra-embryonic part of that vesicle by a 

 process of infolding of somatopleure ventralwards around the 

 embryonic area. This infolding gives rise to certain grooves and 

 folds. The grooves are cephalic, lateral, and caudal. In the chick 

 (to which the following description applies) the cephalic or anterior 

 groove lies, in a crescentic manner, in front of the head-end of the 

 embryo, the concavity of the crescent being directed towards the 

 head. The lateral grooves are disposed one on either side of the. 

 embryonic area. The caudal or posterior groove is situated, in a 

 crescentic manner, behind the tail-end of the embryonic area, the 

 concavity of the crescent being directed towards the tail. By the 

 coalescence of these four grooves one continuous groove is formed, . 

 which surrounds the entire embryonic area, and is known as the 

 marginal or circular groove. 



The folds lie external to the grooves, of which they are the elevated- 

 outer lips, and they are spoken of as head-fold, lateral (right and 

 left), and tail-fold. They consist of two layers of somatopleure, 

 inner and outer, and constitute the amniotic folds. 



As the circular groove deepens, it encroaches, from all sides, upon 

 the ventral aspect of the embryonic area as far as the central part, 

 which coincides with the site of the future umbilicus. The head-, 

 tail-, and lateral amniotic folds become more elevated, the embryo 

 ^inks slightly, and the amniotic folds approach one another over the 

 embryo. Subsequently these folds unite over the dorsal aspect of 

 the embryo, and give rise to a sac, called the amnion, which sur- 

 rounds the embryo. (In man the amnion is formed in a different 

 manner, as will be presently described.) 



After fusion of the amniotic folds has taken place to form the 

 amnion, the inner layer of somatopleure separates from the outer 



