Embryologic Developmerit of Animals 451 









Fig, 224. — Cleavage, blastulation, gaslrulation, and formation of three primary 

 germ layers (ectoderm, mesoderm, entoderm) of mammalian embryo. The early 

 stages have never been actually observed in human beings but probably resemble 

 those of other mammals. 



/, Fertilized cg^ cell; 2, two-celled stage; 3, three-celled stage; 4, morula stage; 

 5, morula stage in half-section, showing outer trophectoderm {T) and inner cell 

 mass (/) ; 6, blastocyst stage in half-section," showing amniotic cavity {A.C.) above, 

 and a single layer of entoderm {En.) ; 7, later blastocyst in median section, show- 

 ing amniotic cavity {A.C.) and yolk sac {Y.S.) separated by a cellular mass consti- 

 tuting the embryonic disc or shield {E.D.), the latter eventually forming the em- 

 bryo. The amniotic cavity is lined with ectoderm {Ec.) and the yolk sac with 

 entoderm {En.) Observe the two types of mesoderm known as the somatic meso- 

 derm {S.M.) and the splanchnic mesoderm {Sp.M.). Two layers of mesoderm 

 form the extraembryonic coelom {E.C.) between them. 8, Blastocyst containing 

 human embryo {E.) about twenty-one days old, showing chorionic villi {C.V.) 

 for securing nourishment; the allantois {A.) \ splanchnic mesoderm {Sp.M.); so- 

 matic mesoderm {S.M.) ; 9, blastocyst containing human embryo about thirty-three 

 days old and about 5.0 millimeters long, showing eye, pharyngeal clefts, front 

 and hind limb buds, tail, and umbilical cord {U.C.). For older embryos see Fig. 

 225. The above stages are represented somewhat diagrammatically; from various 

 sources. 



