124 J- THOS. PATTERSON. 



this entire number between 80 and 90 per cent, of the eggs have 

 given definite results. 



C. Experiments. 



If only the rudimentary somite arises in front of the first 

 mesodermic cleft, an injury made just anterior to this cleft ought 

 to destroy, at least partially, this incomplete somite ; but if, in 

 addition, complete somites are formed anterior to this cleft, the 

 injury ought to appear later in that one of the complete somites 

 lying just in front of the cleft. 



In performing such an operation there are two sources of dif- 

 ficulty. In the first place, owing to the individual variation in 

 the early development of eggs, it is very difficult to hit upon the 

 exact time when but one cleft is present. This difficulty can be 

 met by opening the egg one to two hours before the cleft ordi- 

 narily appears and temporarily sealing the opening, so that it 

 may be reopened and examined from time to time until the cleft 

 appears. It was found that the average time of appearance of 

 this cleft is between 21 and 22 hours. In the second place, it is 

 almost impossible in a large number of embryos, to see the cleft 

 so that one may be sure of the operation. This is due to the 

 fact that the yolk is often a very pale yellow, and the white 

 embryo cannot be seen. However, in about 20 per cent, of the 

 eggs the yolk is a deep yellow, and against this background the 

 embryo stands out in perfect contrast, and one can be absolutely 

 certain of one's operation. In this work only the latter kind of 

 eggs was used. 



Experiment I. 



The operation was performed with an electric needle after the 

 egg had been incubated 22 hours at a temperature of 37— 39 C. 

 Fig. 1 illustrates the place of injury (Fig. 1, 0) and the condition 

 of the embryo at the time of the operation. It will be noted that 

 the first cleft lies just anterior to the fore-end of the primitive 

 streak and meets the main axis of the embryo at an oblique angle. 



After the operation the egg was incubated ten hours. The 

 embryo was killed in picrosulphuric-acetic acid, stained in Conk- 

 lin's picro-hsematoxylin, and mounted in xylol-balsam. 



Fig. 2 shows the result of the operation. On the right side 

 the rudimentary somite is greatly disturbed (Fig. 2, 0) and the 



