2Q2 HELEN DEAN KING. 



from the surrounding tissues. In the head region, the relation of 

 the tissues is practically the same as that shown in Fig. 7. 



When the medullary folds have closed, there is found in the 

 posterior region of the embryo a much narrower dorsal plate 

 than that shown in Fig. 3, as more of the cells have been covered 

 over by the upward growth of the yolk cells from the sides of the 

 archenteric cavity. Anteriorly the dorsal plate grows narrower 

 very rapidly and some distance back of the middle of the embryo 

 the yolk cells have already come to surround the entire archen- 

 teron. By the time that the optic bulbs have formed, there is no 

 longer any dorsal plate in the mid-dorsal wall of the archenteron 

 and the notochord has no connection with any of the surround- 

 ing tissues. 



These results show that the anterior part of the notochord in 

 the embryo of Bufo lentiginosus is entirely mesodermal in origin; 

 in the posterior part of the embryo, the greater part of the noto- 

 chord is also derived from the mesoderm, but there is added to 

 it a single layer of chorda-endoderm from the mid-dorsal wall of 

 the archenteron. Back of the middle region of the embryo, the 

 yolk cells grow up from the lateral walls of the archenteron and 

 unite under the notochord, the cells of the dorsal plate thus cut 

 off from bordering the archenteron, either unite with the noto- 

 chord or are incorporated into the splanchnic mesoderm. 



RANA PALUSTRIS. 



In the frog, Rana palitstris, the notochord is formed at about 

 the same stage of development that it is in Bufo, namely, near 

 the end of gastrulation when the blastopore is closing in. As in 

 the embryo of Bufo, the notochord first appears in the middle 

 region as a rounded chord of cells cut off from the mid-dorsal 

 mesoderm, and it is separated entirely from the ectoderm and also 

 from the endoderm beneath which forms the dorsal wall of the 

 archenteron. At this stage in the development of the egg, the 

 mesoderm in front of the region where the notochord has been 

 cut off forms a solid layer of cells extending across the dorsal 

 wall of the archenteron and entirely separated from it ; the meso- 

 derm back of the notochord also extends in an unbroken sheet 

 across the mid-dorsal region, but in this part of the egg meso- 



