36 



CLEAVAGE OF THE MESOBLAST. 



along the umbilical cord, with the intestine. This is termed the nmWical duct, and the 

 yolk-sac itself has received (in mammals) the name of umbilical reside (figs. 50, 51). 

 Lastly, at the tail end of the embryo a hind-gut is produced. In the human 

 embryo this appears to be formed by a protrusion from the posterior blind end of the 

 enteric groove, and after the formation of the allantoic tube, but in most mammals 



Fig. 37. TRANSVERSE SECTION OP THE' TAIL END OP AN EMBRYO CHICK OF THE LATTER HALP OP 



THE SECOND DAY, AT THE PLACE WHERE THR VERTEBRAL SOMITES CEASE. s ^. (From Kolliker. ) 



m.f., medullary folds, the neural canal beginning to close ; p.m., paraxial mesoblast ; Lm., lateral 

 mesoblast ; ep. , epiblast ; hy, bypoblast ; ao, primitive aorta ; ch, notochord ; p, coelomic cleavage of 

 lateral plate of mesoblast. 



and in birds it is produced by a folding over of the tail end of the embryo like that 

 which occurs at the head end to enclose the fore-gut (figs. 45 to 47). The hind-gut 



Fig. 38. EMBRYO CHICK AT THE END OP THE SECOND DAY, SEEX FROM 

 BELOW. L 5 . (From Kolliker.) 



Vh, forebrain ; Ab, primary ocular vesicles ; Ch, notochord ; //, tubular 

 heart ; om, vitelline veins ; Vd, entrance to the forepart of the alimentary 

 canal within the cephalic fold ; in the middle part of the embryo, the 

 protovertebral somites are seen (to the number of thirteen pairs) on each side 

 of the canal of the spinal marrow and notochord. 



om remains for a considerable time blind, until the anus becomes 

 formed by invagination from the exterior. 



Cleavage of mesoblast. Formation of body cavity. 



At a very early period, soon indeed after the formation of the 

 neural groove, two important changes begin in the mesoblast. 

 One of these is the cleavage of the lateral mesoblast (which is at 

 first a continuous sheet) into two plates, one of which clings to 

 the epiblast, and the other to the hypoblast. The cleft is at first 

 small (fig. 37, p), but accumulation of fluid within it soon con- 

 verts it into a cavity, which gradually spreads until the separation 

 is very extensive (fig. 39, p.p.}. The layer of mesoblast which 

 clings to the epiblast eventually forms part of the body-wall, 

 and is known as the scmatopleure ; that which clings to the 

 hypoblast forms eventually part of the wall of the alimentary 

 tract, and is known as the splanchnopleure. The cavity between these, which is 

 formed by enlargement of the original cleft, is the codom or body cavity (pleuro- 

 peritoneal cavity of authors). 



Formation of mesoblastic somites. The other change occurs not in the 

 lateral but in the paraxial mesoblast, and consists in the occurrence at regular 

 intervals transversely along the mass, of a process of thinning which produces its 

 complete separation into distinct segments, so that when the embryo is viewed from 

 above or below, these segments appear on either side of the neural groove as a 

 linear series of small quadrangular masses (fig. 38), which were originally termed 



