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J. THOS. PATTERSON. 



Experiment II. 



The conditions under which the operation was performed and 

 the subsequent handling of the embryo were the same as in the 

 preceding experiment. Instead of using the electric needle, a 

 very fine glass " pin " was substituted. This pin was placed just 

 in front of the first cleft on the left side at a stage corresponding 

 to that of Fig. I. After the operation the egg was incubated 

 twenty and one half hours longer. 



The result of the operation is shown in Fig. 4. The needle is 

 found in the incomplete somite on the left side (Fig. 4, 0). There 

 are 16 pairs of somites and the heart is well formed. It was im- 



FlG. 3. Sagittal section through the somites on the right side of the embryo repre- 

 sented in Fig. 2. It shows at X the destroyed incomplete somite and at / 

 the place where the needle has broken through the ectoderm. X x 57. 



possible to section this embryo on account of the glass pin, but 

 the surface view is so clear that there can be no doubt as to the 

 position of the pin. 



Experiment III. 



In order to show that the mesoderm lying between the injury 

 and the cleft (between rand a, Fig. 6) does not increase subsequent 

 to the operation and antecedent to the examination of the result 

 and give rise to one or more somites, the injury was made so as 

 to destroy the mesoderm spanning the first cleft (Fig. 6, a). 



The result of such an experiment is well illustrated in Fig. 5. 

 This embryo was treated in exactly the same manner as the one 

 represented in Fig. 2. In addition to destroying the posterior 

 edge of the incomplete somite, the injury extends over a portion 

 of the cleft (Fig. 5, 0). In this, as in Fig. 2, sections confirm 

 what is seen in surface view. 



The above are only a few types out of about seventy- five ex- 



