54 



SEPARATION OF EMBRYO 



constitutes what is known as the septum transversum. The folding-in at the 

 tail end of the embryo takes place rather later, and is complicated by the 



presence of the connecting 

 stalk. In the earliest known 

 human embryos (fig. 79) there 

 is a pocket between the 

 posterior end of the axis and 

 the upper aspect of the stalk. 

 As the embryo increases in 

 length this deepens, the stalk 

 is displaced forwards, and the 



per 



primitive streak is bent in to 

 form, as the anal membrane, 

 the floor of a diverticulum 

 named the hind-gut. In front 

 of the attachment of the stalk 

 the yolk-sac is further folded 

 in and the hind-gut is gradu- 

 ally elongated. Up to this 

 stage the name connecting 

 stalk has been applied to the cord of mesoderm uniting the embryonic rudiment 

 with the chorion. When the tail-fold is produced, it is bent round to the ventral 



F IG . 78. MESIAL LONGITUDINAL SECTION THROUGH THE HEAD 



END OF THE GERMINAL DISC OF THE DOG BEFORE THE 



FORMATION OF THE HEAD-FOLD. (After Bonnet.) 



ect, ectoderm of shield ; not.pl., notochordal plate ; p.p., 

 primitive entodermal plate (Ergcinzungsplatte, Bonnet) ; Ip.m., 

 buccopharyngeal membrane; per, pericephalic portion of 

 pericardial coelom. 



The notochordal plate (archenteric plate, Bonnet) passes 

 directly into the primitive entodermal plate (Ergcinzungs- 

 platte, Bonnet). 





villas 



ammon 



- core of villus 



^jsg^..^ mesoderm 



'connecting stalk 

 'primitive streak 



-yolk-sac 



enloderm 



mesoderm - 



essels 



FIG. 79. MEDIAN LONGITUDINAL SECTION OF AN EMBRYO OF 2 MM. (see FIG. 72). (Graf v. Spee.) 



aspect of the body of the embryo, and may henceforward be appropriately named 

 the abdominal stalk (Bauchstiel, His). 



Between the hind-gut and the fore-gut there is at first a wide opening into the 

 yolk-sac (fig. 92), which is gradually reduced to a narrow aperture, and the stalk 



