CH. LIX.] 



EAELY FCETAL STRUCTURES 



885 



S.O 



and laterally. It looks as if some constricting agent had been placed 



round the margin of the area, and that afterwards the area above 



the constriction, and the area below had 



gone on growing rapidly. In this way, 



the embryo is clearly separated into two 



parts, an upper, the foetus, and a lower, 



which becomes the appendages of the 



foetus.* The anterior part of the folded 



embryonic area is known as the head fold, 



the posterior as the tail fold, and the two 



are connected together on each side by the 



lateral folds. 



The portion of the archenteron en- 

 closed in the foetus forms the primitive 

 gut. The part contained in the head fold 

 is the fore-gut, that in the tail fold is the 

 hind-gut, and the remainder is the mid- 

 gut (fig. 572). 



The constriction where the body of the 

 foetus becomes continuous with the re- 

 maining structures, is known ultimately 

 as the umbilicus. It remains pervious till 

 birth, and through it the mid-gut is 

 connected with the remainder of the 

 archenteron (which is henceforth called 

 the yolk-sac) by a narrow hypoblastic 

 tube, the vitello-intestinal duct (fig. 571, 

 10). 



The portion of the mesoblastic cavity 

 enclosed in the foetus is called the body 

 cavity. It gradually differentiates into the 

 pericardial, pleural and peritoneal cavities, 

 which are eventually entirely separated 

 from one another. 



Before the neural groove is closed and 

 becomes the neural canal, the hypoblast 

 beneath the middle of the groove becomes 

 thickened to form a longitudinal ridge 

 (fig. 569). This ridge is the notochord or 



pr 



FIG. 570. Embryo chick (86 hours), 

 viewed from beneath as a trans- 

 parent object (magnified), pi, Out- 

 line of pellucid area ; FB, fore-brain, 

 or first cerebral vesicle : from its 

 sides project op, the optic vesicles ; 

 SO, backward limit of somatopleur 

 fold, "tucked in" under head; 

 a, head fold of true amnion ; a', re- 

 flected layer of amnion, sometimes 

 termed " false amnion " ; sp, back- 

 ward limit of splanchnopleur folds, 

 along which run the omphalo- 

 mesenteric veins uniting to form 

 h, the heart, which is continued 

 forwards into la, the bulbus arte- 

 riosus ; d, the fore-gut, lying behind 

 the heart, and having a wide cres- 

 centic opening between the splanch- 

 nopleur folds ; HB, hind-brain ; 

 MB, mid-brain ; pv, protovertebrae 

 lying behind the fore-gut ; me, line 

 of junction of medullary folds and 

 of notochord ; ch, front end of noto- 

 chord ; vpl, vertebral plates ; pr, 

 the primitive groove at its caudal 

 end. (Foster and Balfour.) 



* Among many anatomists and obstetricians 

 the word ovum is applied to the foetus and its 

 appendages. It should be properly restricted to 

 the female gamete. After development has com- 

 menced the term embryo should be employed, and that word includes the embryo 

 proper which we call the foetus, and its appendages. 



