132 J. THOS. PATTERSON. 



I have shown that this author has mistaken the most posterior 

 of certain transitory shallow depressions in the head mesoderm 

 for a cleft. I have also made it clear that the first few (2 or 3) 

 clefts are not completed until six or seven pairs of somites are 

 formed. It seems reasonable to suppose, therefore, that Miss 

 Piatt has interpreted the first cleft, during the early stages of 

 development, as a derivative of the most posterior shallow de- 

 pression. It should be added that this posterior shallow depres- 

 sion persists longer than the others. 



Since these shallow transitory depressions are situated at regu- 

 lar intervals, I might suggest that they lend themselves to another 

 interpretation, namely, as vestigial clefts separating the cephalic 

 mesoblastic somites. Notwithstanding the fact that Locy ('95) 

 and his followers minimize the value of myotomes in ascertaining 

 the metamerism of the vertebrate head and use neuromeres as the 

 sine qua non for determining primitive segmentation, nevertheless 

 the glimpses one gets of such structures as that cited above 

 should not be overlooked. In fact, if these vestigial clefts are 

 studied in connection with the various conditions seen in the 

 myotomes, the above interpretation becomes evident, for in pass- 

 ing backwards from the anterior end of the embryo one finds that 

 the clefts become more and more pronounced. This is evidenced 

 by (1) the vestigial clefts, (2) the rudimentary somite, whose 

 anterior cleft fails to separate it from the head mesoderm, (3) the 

 slowness of the first clefts in cutting off the anterior protoverte- 

 brae, (4) and finally the sharpness and rapidity with which all 

 succeeding protovertebrse are cut off. In other words the influ- 

 ence of the process which has completely obliterated or greatly 

 modified the anterior cephalic somites, gradually becomes weaker 

 in passing posteriorly, and finally ceases altogether. 



In regard to the experimental work it seems unnecessary to add 

 to what has already been said. It was noted that an injury made, 

 either with a glass pin or an electric needle, just anterior to the 

 first cleft, appeared upon further incubation, in the rudimentary 

 somite, showing beyond a shadow of doubt that no somite, ex- 

 cept the incomplete one, is formed anterior to the first mesoder- 

 mic cleft. This brings the order of the appearance of somites of 

 the chick into harmony with the general law for the early devel- 



