536 



TEXTBOOK OF ZOOLOGY 



processes of gastrulation proceed, and soon it takes up the space for- 

 merly occupied by the cleavage cavity or blastocoele. Its posterior 

 end is the blastopore, and this, as previously mentioned, is plugged 

 with yolk or vegetal cells Imown as the yolk plug. As a final result, 

 the gastrula forms a two-layered embryo of ectoderm and endoderm 

 cells, each layer of which may be several cells thick. 



Mesoderm Formation. — Before the process of gastrulation is com- 

 pleted, a sheet of cells forms between the ectoderm and endoderm 

 cells. This sheet of cells is known as mesoderm. As the mesoderm 

 grows it splits into two sheets, an outer, or somatic layer, which lies 

 next to the ectoderm of the embryo wall, and an inner, or splanchnic 



otic pit 



ectoderm 



mural tube 



mesoderm 



notochord 

 hypochord 

 .sonvaic mesoderm 

 splanchnic mesoderm-^ 



midjut- 



fc>rej;ut 



^oik 



<Seciion through oiic pit 



^eciion ihrou^h mid-^ut 



Fig. 290. 



FROGL EMBRYO, SflCfWltia nMODER.'A rOLD.5 



-Two levels through the body of the neural tube stage of the developing 

 frog. (Courtesy of General Biological Supply House.) 



layer, which lies next to the endoderm cells of the archenteron. The 

 cavity formed between these two layers is the beginning of the coelom, 

 or body cavity. From these three layers, ectoderm, endoderm, and 

 mesoderm, all the body structures are formed. 



Formation of Nervous System. — With the reduction of the blasto- 

 pore to a very small area, there appears on the dorsal side of the 

 embryo a thickened plate of ectoderm known as the neural plate 

 which is wide in front but narrows posteriorly. This plate soon is 

 flanked on each side and in front by ridges known as neural folds. 

 As these folds arise, the remainder of the plate becomes a longitudinal 



