12 



Hybridization of Echinoids. 



classes of Echinoderms. To the list given in the paper cited should 

 be added Mortensen's contributions on Crinoids and that of Oshima 

 (1918, 1921) on the Holothurian Cucumaria echinata, in which the 

 mesenchj^me cells are described as migrating into the blastocoele 

 before the beginning of gastrulation. 



Fig. 5a, Skeletal rods from fixed and mounted Cidaris larva. 6 days. X 350. 

 56, Cidaris larva, 2 weeks old, fixed and mounted. Camera sketch. X 110. 

 5c, Camera sketch of living Cidaris 9 X Lytechinusc^ gastrula. 24 hours. X 220. 

 5d, Camera sketch of living Cidaris 9 X Lytechinus cf gastrula. 24 hours. X 220. 

 5e, Camera sketch of living Cidaris 9 X Lytechinus d* gastrula. 40 hours. X 220. 



Mortensen's (1920) studies have shown that there is diversity in 

 the manner of formation of mesenchyme in the Crinoids, a fact 

 which our knowledge of the development of three species of Antedon 

 had failed to reveal. These observations do not affect the conclusions 

 that I have drawn regarding the primitive nature of the development 

 of Cidaris. 



Cidaris tribuloides, in the estimation of those qualified to speak on 

 the systematic position of echinoids, is primitive. This fact suggests 



