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AN INTRODUCTION TO ZOOLOGY 



from it, giving rise to a more definite layer that first appears in the 

 dorso-lateral regions. The presence of the yolk-laden cells in the 

 ventral half of the embryo delays matters somewhat, but cells are 

 given off by the dorsal mesoderm which pass further and further 

 ventrally and form a more or less continuous layer of mesoderm 

 that separates the yolk-laden entoderm from the ectoderm. In the 

 actual mid-dorsal line, particularly near and in front of the blast opore, 

 the mesodermal cells are slightly differently related to the other 

 layers and the formation of the notochord leads to certain modifi- 

 cations. 



Along the axial line the rudiments of the notochord, the 

 mesoderm, the dorsal entoderm, and to a certain extent also the 



ectoderm, are not 

 clearly separable and 

 form an axial mass, 

 while laterally to that 

 the mesoderm is sepa- 

 rated from both ento- 

 and ectoderm. As the 

 blastopore closes the 

 slit between ecto- and 

 mesoderm on each side 

 passes inwards towards 

 the middle line, but 

 before reaching it turns 

 downwards and stops. 

 This leaves a thickened 

 layer of ectoderm (the 

 neural plate, vide in- 

 fra), superficially and 

 beneath it a sort of 



wedge-shaped vertical mass of cells, the notochord rudiment now 

 cut off from the lateral mesoderm. The cleft separating ento- 

 and mesoderm also passes inwards, but stops short of the middle 

 line below and slightly lateral to the other slit, thus leaving the 

 entoderm cells continuous right in the middle line with the 

 notochordal cells and just lateral to this continuous with the 

 lower portion of the lateral mesoderm mass, by this time fairly 

 thick. Along the regions where the dorsal entoderm is con- 

 tinuous with the mesoderm in this way, shallow but nevertheless 

 distinct grooves appear which are taken to represent enterocoelic 

 invaginations homologous with those in Amphioxus. At a later 

 stage the appearance of a slit cuts the notochord off from the ento- 

 derm leaving only a single layer of its cells to roof the enteron, and a 



FIG. 133. Ranafusca, transverse section through 

 young embryo. -After O. Hertwig. 



A., archenteron ; E., ectoderm ; E.G., traces of enterocosls ; 

 En., entoderm ; M., mesoderm ; N., notochord ; N.P., neural 

 plate ; Y., yolk cells. 



