52 SEPARATION OF EMBRYO 



The cleavage first makes its appearance at the anterior end of the axis in the 

 region where the heart-tubes will be formed. Thence it extends backwards, and 

 at the same time forwards round the head end of the axis, so that the lateral 

 coelomic spaces are continuous with one another in front, by a pericephalic cleft 

 which afterwards becomes the pericardium. 



The relations of the layers immediately in front of and behind the axis must 

 finally be referred to. In the axial line the notochord passes in front into the head- 

 plate. If this be followed forwards (fig. 78), it will be seen that it is continued into 

 a portion of the blastoderm between the head end of the axis and the pericephalic 

 ccelom, into which the mesoderm has not extended (or from which it has 

 disappeared). The ectoderm and entoderm are therefore here in contact, and form 

 a membrane known as the buccopharyngeal membrane, which later becomes 

 perforated to form the mouth-opening. The region of the blastoderm between the 

 buccopharyngeal membrane and the edge of the shield corresponds to the ' pro- 



volk-xic 



^ amnion 



neurenteric canal 

 _1 allantoic diveriicnlum 



FlG. 75. SUBFACE VIEW OF A BLASTODEEM OF CEBCOPITHECUS CYNOMOLGUS. (After Seleilka.) 



The amnion has been opened. The first three segments are visible in front of the neurenteric 

 canal on each side of the neural groove, which is still open. 



amnion ' of lower mammals ; but in the human embryo the ectoderm and entoderm 

 are, from the first, here separated by mesoderm. This is not split, however, so that 

 the pericephalic is separated from the extra-embryonic coelom by a bridge of tissue. 

 Again, at the posterior end of the axis, behind the growing point or tail-knob, the 

 primitive streak becomes detached from the lateral mesodermic sheets and resolved 

 into an ectodermal and an entodermal lamella, which together form the cloacal 

 membrane. This is afterwards perforated to form the urogenital and anal apertures. 

 Separation of the embryo : history of the yolk-sac and allantois. 

 As the embryo increases in length, there is a certain increment also in the breadth 

 of the embryonic shield ; and although the yolk-sac has much increased in size, 

 the embryo soon begins to expand in all directions beyond the limits of the mouth 

 of the sac. A folding-in round the margin of the shield, along the line where 

 amniotic and embryonic ectoderm meet, consequently takes place. The anterior 



