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W. M. BALDWIN. 



of the head region. Actual continuity of the tissues, however, 

 does not exist. The cephalic end of the embryo, which is repre- 

 sented below and to the left of the sketch, is distinguished by its 

 greater transverse diameter. The area of protoplasm in this 

 specimen affected by the rays lay in the concave bend of the 

 trunk at the extremity of the cleft. It will be noted that this 

 flexure is sharply restricted to a relatively small portion of the 

 embryo. Cephalic to and caudal to this level, both the head-end 

 and the tail-end of the embryo are, respectively, of normal shape. 

 Occasionally, the two neural tube-halves fail to unite in the 

 caudal region, bringing about a condition which is demonstrated 

 well by Fig. 3. In this the yolk mass is represented as projecting 



FIG. 3. 



to the right between the caudal extremities of the divided neural 

 tube. This is an early developmental feature in the production 

 of the condition of spina bifida, the divided condition of the 

 neural tube-halves persisting in the adult free-swimming tadpole. 

 The bend in this particular embryo is represented as occurring 

 high up in the trunk region. The cephalic end of the embryo is 

 shown with its right surface resting against the right neural 

 tube-half caudally. As was noted in connection with the pre- 

 ceeding figure, so is it likewise true of this, both cephalic to 

 and caudal to the angle of the fold, aside from the features of 

 spina bifida, the appearance of the embryo is practically normal. 

 The stages of development represented by these two figures are 

 relatively early, consequently but little of the ordinary features 

 of external configuration of the embryo can be made out. 



