188 



EMBRYOLOGY. 



the formation of the amnion, the yelk-sac forms a blind pouch which 

 is lined by hypoblast and has an external covering of mesoblast 

 (Fig. 59), as is shown at 6 in the accom- 

 FIG. 59. panying cut. The upper 6 is in the primi- 



tive gut, which derives its epithelial ele- 

 ments from the hypoblast and its muscular 

 and serous coverings from the mesoblast. 



What is the yelk-sac then called? 

 The umbilical vesicle. 



What is the function of the umbilical 



vesicle ? 



From it the embryo derives its suste- 

 nance in the earlier stages. Early in the 

 development blood-vessels begin to form, 

 and they ramify over the surface of the 

 umbilical vesicle and help to absorb its 

 contents. 



Transverse Sections through 

 the Embryo Chick, before 

 and some time after the 

 closure of the medullary 

 canal, to show the upward 

 and downward inflections 

 of the blastoderm (after Ke- 

 mak) on the third day in 

 the lumbar region : 1, noto- 

 chord in its sheath ; 2, me- 

 dullary canal, now closed 

 in ; 3, section of the medul- 

 lary substance of the spinal 

 cord; 4, corneous layer; 5, 

 somatopleure of the meso- 

 blast; 5', splanchnopleure 

 (one figure is placed in the 

 pleuro-peritoneal cavity) ; 6, 

 layers of hypoblast in the 

 intestines, spreading also 

 over the yelk ; 4X5, part ot 

 the fold of the amnion form- 

 ed by epiblast and somato- 

 pleure. 



What is the amnion? 



While the one portion of the split meso- 

 blast (splanchnopleure) unites with the 

 hypoblast to form the splanchnoblast and 

 alimentary organs, the outer layer (soma- 

 topleure) and epiblast are united to form 

 the skin and walls of the body as the soma- 

 toblast. The somatoblast now folds in 

 about the embryo. It must be remem- 

 bered, however, that the entire globe of 

 the ovum is lined with these cells, and 

 that in folding in this way the cells are in 

 a measure raised away from the wall of the ovum. The layers of 

 the somatoblast fold back until they meet and unite behind the 

 embryo, and in this way form a layer of membrane which lines 

 the ovum and another layer which encloses the embryo (Fig. 60). 

 This latter layer forms the amnion. (This is better understood 

 by reference to the cuts.) 



What is the use of the amnion ? 



It covers the embryo in the early stages very closely, but soon 

 becomes distended with a pale watery fluid, and serves to float 



