FORMATION OF NOTOCHORD IN AMPHIBIA. 



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Transverse sections through an embryo in which the medul- 

 lary plate has just appeared show that, in the anterior region, 

 the notochord is composed of a rounded mass of cells cut off 

 entirely from the surrounding tissues, and appearing much as in 

 Fig. 7. In the posterior region, there is, as yet, no trace of a 

 notochord, and an unbroken layer of cells extends across the 

 dorsal surface of the archenteron, as here the mesoderm is still 



FIGS. 3-7. Serial sections from the posterior to the middle region of an embryo 

 of Bufo lentiginosus in which the medullary folds are closing. 



united with the endoderm as in the earlier stages. In a section 

 made a short distance behind the middle of the embryo (Fig. 2), 

 the notochord appears as a triangular shaped chord of cells, 

 entirely distinct from the mesodermal layer on either side, but 

 closely connected with the cells forming the mid-dorsal wall of 

 the archenteron. In this part of the embryo, as well as more 



