CHAPTER XIV 



ORGANS FROM THE MESODERM 



The mesoderm appears as a distinct layer over the dorsal 

 surface of the embryo at the time when the dorsal lip of the 

 blastopore is moving over the white hemisphere (Fig. 25). 

 At first the mesoderm is in close contact with the endoderm, 

 particularly along the mid-dorsal line. The notochord soon 

 separates from the mesodermal sheets of each side by two verti- 

 cal furrows, so that from this time forward there are two lateral 

 sheets of mesoderm, separated in the mid-dorsal line by the 

 notochord (Fig. 26, E). Around the anterior and posterior 

 ends of the notochord, the two sheets of mesoderm are con- 

 tinued into each other. 



These sheets of mesoderm now rapidly extend ventrally. ( 

 This down-growth is brought about by additions to the ven- 

 tral borders of the sheets. The new cells that are added 

 come, probably, from the yolk-cells along the free borders of 

 the mesoderm ; the yolk-cells in this region dividing rapidly 

 form smaller cells that are joined to the mesoderm. 1 At the 

 time when the medullary folds appear outlined upon the sur- 

 face, the lateral sheets of mesoderm have extended ventrally 

 and to a certain extent have fused in the mid- ventral line. 

 The cells of each sheet of mesoderm are arranged over the 

 greater part of their extent into two layers ; but on each side 

 of the notochord the mesoderm is somewhat thickened to form 

 the beginning of the segmental plate (Fig. 42) ; and in this 

 region there is, in the early stages of development, no distinct 

 arrangement of the cells into two layers. 



1 According to some authors the ventral extension of mesoderm results from 

 a proliferation of the mesoderm that is first laid down over the dorsal region, 

 but it seems to me there is little ground for such an assumption. 



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