SEPARATION OF THE EMBRYO 



53 



be arranged in the following groups, viz.: cervical 8, thoracic 12, lumbar 5, 

 sacral 5, and coccygeal from 5 to 8. Those of the occipital region of the head 

 are usually described as being four in number. In mammals primitive segments 

 of the head can be recognized only in the occipital region, but a study of the 

 lower vertebrates leads to the belief that they are present also in the anterior 

 part of the head, and that altogether nine segments are represented in the 

 cephalic region. 



SEPARATION OF THE EMBRYO. 



The embryo increases rapidly in size, but the circumference of the embryonic 

 disk, or line of meeting of the embryonic and amniotic parts of the ectoderm, is of 

 relatively slow growth and gradually comes to form a constriction between the 

 embryo and the greater part of the yolk-sac. By means of this constriction, which 

 corresponds to the future umbilicus, a small part of the yolk-sac is enclosed within 

 the embryo and constitutes the primitive digestive tube. 



Villi of chorion 



Amnion 

 Embryonic disk 



Rudiment of heart 



Chorion 

 Mesoderm, 



Body-stalk 

 Primitive streak 



Mesoderm 



Bloodvessel 



Fia. 21. Section through the embryo which is represented in Fig. 17. (After Graf Spec.) 



The embryo increases more rapidly in length than in width, and its cephalic and 

 caudal ends soon extend beyond the corresponding parts of the circumference of 

 the embryonic disk and are bent in a ventral direction to form the cephalic and 

 caudal folds respectively (Figs. 26 and 27). The cephalic fold is first formed, and 

 as the proamniotic area (page 47) lying immediately in front of the pericardial 

 area (page 47) forms the anterior limit of the circumference of the embryonic 

 disk, the forward growth of the head necessarily carries with it the posterior end 

 of the pericardial area, so that this area and the buccopharyngeal membrane are 

 folded back under the head of the embryo which now encloses a diverticulum of the 

 yolk-sac named the fore-gut. The caudal end of the embryo is at first connected 

 to the chorion by a band of mesoderm called the body-stalk, but with the formation 

 of the caudal fold the body-stalk assumes a ventral position; a diverticulum of the 

 yolk-sac extends into the tail fold and is termed the hind-gut. Between the fore-gut 



