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HELEN DEAN KING. 



distinct from the ectoderm, but it is united for some distance with 

 the cells forming the dorsal wall of the archenteron. At this stage 

 of development there is first noticed, in the middle of the em- 

 bryo, a pronounced thickening of the mid-dorsal mesoderm (Fig. 

 I, yV), which extends only over a few sections at first and is con- 

 tinuous with the lateral mesoderm on either side. When the 

 blastopore is nearly closed, the thickened portion of the mesoderm 

 is cut off from the lateral mesoderm to form the notochord, the 

 line of separation coming in at about the points marked XX in 

 Fig. i. As the embryo elongates, the forward extension of the 



~FlG. I. Part of a medium sagittal section through an egg of Btifo lentiginoszts 

 in which the blastopore has begun to close. A 7 , thickening of mid-dorsal mesoderm 

 which is to be cut off at the points XX to form the notochord. 



FIG. 2. Part of transverse section through the posterior region of an embryo j n 

 which the medullary plate has appeared. 



notochord always takes place in this same way, /. c., by the cut- 

 ting off, laterally, of a portion of the mesodermal layer in the 

 mid-dorsal region so that, from the beginning, the notochord is 

 entirely separated from the ectoderm and also from the endoderm. 

 These observations confirm the statement made in a previous 

 paper (King, 12) that " the anterior part of the notochord is cer- 

 tainly mesodermal in origin." 



