108 



DEVELOPMENT OF THE FROG'S EGG 



[Cn. XI 



the right half of an embryo has developed from the uninjured 

 blastomere. Half a medullary plate is present along the line 

 of separation of the injured and uninjured halves. Near the 

 posterior end of the half plate, the yolk of the developed half 

 is exposed over a small region and surrounded by half of a 

 blastopore (?). A cross-section of such an embryo shows 

 (Fig. 35, B) that the half plate has essentially the same form 

 as half of the normal medullary plate ; that beneath this half 

 plate a notochord is present forming a rod, round or slightly 

 oval in cross-section ; that a small archenteron is present in 

 the developing half, and that a mesodermal sheet is present 

 over the side of the hemiembryo. It is interesting to note 

 that while only half the medullary plate is present, yet the 



FIG. 35. Cross-sections through two half-embryos of different stages. (After Roux.) 



notochord and archenteron, which are also median structures, 

 form whole structures but of smaller size than the correspond- 

 ing normal organs. Roux thought that the notochord was 

 very probably composed of only half the number of cells pres- 

 ent in the normal notochord, but, OAving to a great amount of 

 variation in the latter, it was not possible to determine this 

 relation definitely. 



Pfliiger, Roux, and Born have shown that sometimes in the 

 normal development the plane of the first cleavage corresponds 

 to the cross-plane of the body of the embryo, i.e. the plane of 

 the first cleavage separates the anterior from the posterior end 

 of the body. Under these circumstances, if one of the first two 



