262 PRINCIPLES OF EMBRYOLOGY 



pocket, to be mentioned later, is formed under the tail, originating at a 

 later stage than the foregut. It slowly extends forwards, and the two 

 gradually come together near the middle of the body, leaving a narrow 

 coimection through which the gut opens out on to the underlying yolk; 

 this is the umbilical cord. It is not till near the end of incubation that the 

 chick gut becomes completely closed, and this is achieved, not by closing 

 off the umbilical cord, but by the small remaining piece of yolk being 

 drawn in inside the body, when the ectoderm and mesoderm close the 

 hole through which it has entered. 



6. The trunk: mesodermal structures 



The mesoderm makes up the greater part of the bulk of the adult 

 body, between the outer ectodermal layer of the skin (the epidermis) and 

 the iimer endodermal layer of the gut (the intestinal lining). 



The first organs wliich become separated out from the rest of the meso- 

 derm are those lying along the dorsal side. Immediately under the centre 

 of the neural tube, a long rod is formed; this is the notochord, which 

 acts as the first element in the skeleton, providing a longitudinal stiffening 

 of the axis of the body. Its stiffness is not due in any large part to the pro- 

 duction of hard or inflexible substance, but to the fact that the cells be- 

 come swollen with fluid, so that the whole rod becomes turgid and stiff. 

 The principle is one which Nature has made use of in other cases, where a 

 temporary stiflhess is required for physiological functioning. In this case 

 the turgor is more permanent since the notochord induces the neighbour- 

 ing mesoderm to secrete around it a thin but inelastic sheath (Mookerjee 



1953). 



On each side, the layer of mesoderm is thicker at its median edges, 

 where it abuts on to the notochord. Very soon, this thicker portion 

 becomes more or less separated from the more lateral parts by the forma- 

 tion of a thinner longitudinal strip; this is known as the intermediate 

 mesoderm, and from it the kidneys will develop. The thicker medial 

 strips of mesoderm soon become cut up by a series of transverse grooves 

 into separate blocks, known as the somites, which lie in pairs on each side 

 of the notochord. The transverse grooving, and thus the appearance of 

 the distinguishable somites, begins at the anterior end, and gradually 

 progresses posteriorly, until there may be forty or more pairs of somites; 

 the number differs in different species. One or more of the most anterior 

 pairs may break up and disappear fairly shortly after their formation. The 

 remainder of the somites persist and gradually give rise to the main 

 segmental organs of the trunk, particularly the vertebrae and the associa- 

 ted segmental muscles ; they also contribute to the dermal layer of the 



