538 HERRICK E. WILSON 



If the absorption or atrophy of one side of a plate and the 

 growth on the opposite side are approximately equal, the plate 

 would appear to be shifting as a whole, although actually stationary 

 in part. This type of migration may be called portional migration, 

 and is the type illustrated in the shifting forward of the postero- 

 lateral basals in Xenocrimis and sometimes in Encaly ptocrinus and 

 Callicrinus. 



One form of cell-group migration involving the approximation 

 and fusion of two groups into one would occur, if in the previous 

 development of phylitic compression of characters anchylosis were 

 to be carried back into the embryonic period. Thus, anchylosis 

 which appeared as an adult character in early times might appear 

 as an embryonic character in a later stage of development, and 

 cause fusion of the formative cell groups. It is suggested that this 

 type of fusion might be responsible for the interradial development 

 of the two larger infrabasals in Antedon. 1 That cell groups as such 

 may migrate in response to physiological stimuli from changed 

 environment without any such evolutional change is also possible, 

 and experimental evidence has been obtained to substantiate this 

 hypothesis. 2 



Simple migration, after plate formation, without tortion or 

 other movement of the sarcode, seems from the very nature of 

 plate growth (see p. 501) to be impossible. This opinion is appar- 

 ently substantiated by the system of migration of the anal plate in 

 Antedon, and of the radianal in Promachocrinus 3 and Hathrometra, 4 

 and by the development of the posterior radials in Antedon upon 

 the introduction of the anal. Equal spacing of the radials and the 

 anal in the hexagonal stage of Antedon is not apparent; on the 

 contrary, the space separating the postero-lateral and antero-lateral 

 radials is much greater than that separating the postero-lateral 

 radials and the anal. As the plates increase by branching and 

 anastomosis, the adjacent margins of the anal and the postero- 

 lateral radials meet and assume a finished appearance (Fig. 10) 

 before the postero-lateral and antero-lateral radials meet. For- 

 ward shoving of the postero-lateral radials into this unoccupied 



1 Ref. 8, pp. 288, 289. ^Ref. 16, p. 332. 



2 Ref. 26, p. 90. «Ref. 24, pis. 8-12. 



