GERM LAYERS. 89 



81). The splitting of the mesoderm begins some distance from the medial line 

 and proceeds both laterally and medially, but does not extend quite to the 

 primitive segments. Thus a solid plate of cells still remains between the coelom 

 and the segments the intermediate cell mass (Fig. 81). The ccelom shows no 

 segmentation. (Compare Fig. 80 with Fig. 74 and Fig. 81 with Fig. 72.) 



The formation of the neural groove and tube from the ectoderm and the 

 separation of the chorda from the entoderm are processes quite analogous to the 

 development of those same structures in the lower forms. 



As in the chick, so also in Mammals, the blastoderm is at first spread out flat, 

 forming the roof, so to speak, of the yolk sac. At a later period, in connection 

 with the closure of the gut and the establishment of the external forms of the 

 body, the blastoderm assumes a tubular shape (see p. 140). 



A comparison of the foregoing description of the formation of the mesoderm 

 in Mammals with the description of the corresponding processes in the chick' 

 (p. 79) shows their essential similarity. 



Strand of mesoderm 

 in exocoelom 



Entoderm 

 of yolk sac 



Mesoderm 

 of yolk sac 



/s 



Part of exocoelom 



Trophoderm 



Mesoderm of chorion 

 Ectoderm of amnion 

 Entoderm 

 Amniotic cavity 

 Embryonic ectoderm 

 Mesoderm 

 Yolk cavity 



Mesoderm 



FIG. 82. Section through human chorion, arryiion, embryonic disk, and yolk sac. Peters. 



Compare with Fig. 83. 



The Germ Layers in Man. 



Of the actual formation of the germ layers in man, practically nothing is 

 known. There are no observations on the segmentation of the ovum, the first 

 differentiation of cells, or the origin of the embryonic disk and germ layers. 

 A very young human ovum, described by Leopold, does not show any structures 

 which can be interpreted as the embryonic disk or any part of it. Another 



