No. 3.] ON- INTERCALATION- OF VERTEBRA. 335 



men the first caudal is the thirty-sixth {Heloderma horriduvi) \^ 

 in the second, the thirty-seventh {Hcloderma horridum, observed 

 by me); in the third, the thirty-eighth 2 {Helodcrma suspectum); 

 in the fourth it is the thirty-ninth {Helodcrma suspectiim, Clark 

 University). We have, therefore, four variations in four speci- 

 mens. There seems to me very little doubt that this difference 

 in the number of vertebras is produced by true intercalation. ^ 



By these few but characteristic examples I believe to have 

 given positive evidence that intercalation of segments takes 

 place in vertebrates. I do not doubt that further examination 

 of more material will bring out more cases. What is necessary 

 to do, is to examine a great number of specimens of the same 

 and allied species of such forms as show an unusual increase 

 of segments, like the Varanidae, Scincidse, Anguidse, Amphis- 

 baenidae, Snakes, and so on. The embryology of such forms 

 would probably give important evidence, because we may expect 

 to find indication of myomeric division. 



My opinion is that in the iticrease of the number of seo-ments 

 not only in vertebrates, b^it also in invertebrates, intercalatioii has 

 played a vmch greater role than is generally admitted. At the 

 same time I admit addition of segments at the distal end, as well 

 as occasional sligJit migration of the shoulder girdle and pelvis 

 in both directions. The question is an important one, and 

 I hope that some embryologist may take up the subject for 

 further study. 



This question of increase of the number of segments is a 

 very interesting one from the standpoint of the evolutionist. 

 It is evident that intercalation can only take place in the very 

 early life of the embryo, when the myotomes are forming, and 

 that it is absolutely impossible that new segments can be 

 intercalated through any effort and exercise of the animal 



1 Troschel, F. H. : Uber Heloderma horridiim. Wiegm. Arch. f. Naturg., Jahrgang 

 19, Vol. I, Berlin, 1853, pp. 294-315. I am indebted to Mr. S. Garman, Cambridge, 

 for looking up this reference for me, the Journal not being at hand. 



■■^ Shufeldt, R. W. : Contributions to the Study of Heloderma suspectum. Proc. Zool. 

 Soc, London, 1890, p. 214 (the peculiar character of the first caudal is not mentioned 

 by Dr. Shufeldt). 



3 Whether this intercalation is produced by division of myotomes, or by addition 

 of myotomes from the beginning, I do not know; both ways are possible. I am 

 convinced that in a great number of cases intercalation takes place by adding new 

 segments in the embryo without division. 



