428 MAMMALIA. 



thickened axial portion of hypoblast. The area in which 

 these three layers arise is the embryonic area, and here alone 

 the foetus is formed. The other part of the egg forms the 

 umbilical vesicle (fig. 203, UV), which folds off from the embryo 

 and may be likened to the yolk-sac of the fowl's egg. But in 

 this sac we do not find yolk but a fluid that readily coagu- 

 lates. The umbilical vesicle becomes surrounded by hypoblast, 

 just as the yolk-sac of the bird became encircled by it. 



About eight days after impregnation there may be seen a 

 splitting of the lateral mesoblast into a vertebral and peripheral 

 zone, and in the former two somites appear. At the ninth day 

 a clear ring is seen around the embryo, and outside this an area 

 vasculosa. The medullary plate swells into a cephalic enlarge- 

 ment. On each side of the head is to be seen a long tube. 

 These two tubes are the commencement of part of the heart. 

 Later the trunk of the embryo grows faster than the head, 

 and the head becomes folded off, and a little later the tail- 

 fold appears. The medullary groove remains open for some 

 time, and forms the fore-brain, mid-brain, hind-brain, and optic 

 vesicles by a series of enlargements. At the close of the ninth 

 day the medullary groove becomes closed and the head 13 quite 

 free, the two halves of the heart coming together and form- 

 ing a median single tube. The somites also increase to about 

 twelve. During these changes the amnion (fig, 203, am) has 

 nearly encircled the foetus, and the allantois (al) has commenced 

 to grow as in birds. Later development consists of the well- 

 marked cranial flexure and the presence of three visceral arches. 

 The first forms the posterior border of the mouth, and sends 

 forward a process on each side which forms the anterior margin 

 of the mouth. Although there are only three arches, there 

 are nevertheless four visceral clefts. The whole embryo be- 

 comes very much bent, the limbs appearing as buds in the 

 same way as in the chick. 



The fcetal membranes in mammals differ much from those of 

 birds, except in the early stages. Mammalian foeti are closely 



