i 3 o 



J. THOS. PATTERSON. 



falls by using such an index, becomes apparent on examination of 

 sections such as are represented in Figs. 9 and 10. In Fig. 9, 

 clefts a, b, and c are so nearly alike that it would be impossible 

 to say which is the oldest, judging from their depths. 



In Fig. 10 cleft b is slightly deeper than a, but c is still deeper. 

 In such cases, those who accept the relative depths of the clefts 

 as a criterion for drawing conclusions, would be forced to say 

 that two and one half somites arise anterior to the first formed 

 somite, because, as Miss Piatt correctly states, " the first cleft lies 



Figs. 9 and 10. Sections of three and four somites respectively. Both figures 

 show the failure of the first clefts to cut off completely the anterior somites in the 

 beginning. X *57- 



anterior to the first protovertebra, not posterior, as Kupffer and 

 Benecke supposed." 1 In other cases I have observed the fourth 

 cleft to be the deepest. 



The condition seen in Fig. 10 is a very common one, and is 

 brought about by the manner in which somites posterior to the 

 first two or three are cut off. In the beginning the first clefts 

 never completely separate their bordering somites, so that the 

 anterior somites often remain connected until the sixth or seventh 

 pair is formed, but posterior to these anterior clefts, succeeding 

 somites are delimited, often before there are any indentations on 



1 Loc. cit., p. 176. 



