Hyhridizaiion of Echinoids. 11 



solid mass, but they form instead the inner end of the hollow archen- 

 teron. The compact mass of cells shown by Prouho in his figure 

 causes me to believe that the manner of formation of mesenchyme in 

 Dorocidaris papillata will be found, on reexamination, to be of the 

 type that I have described for Cidaris iribuloides. 



Mortensen (1921) has described observations that he made in 

 1915 on Eucidaris thouarsi. His figure of a larva 6 days old dem- 

 onstrates that the thouarsi larva is very similar to the Iribuloides larva. 

 Mortensen (1921, p. 78 et seq. and appendix) has suggested that the 

 very unusual larva that he has described as Echinopluteus transversus 

 is really the Cidaris larva. The additional evidence presented in 

 this paper supports, so far as it goes, Mortensen's suggestion. My 

 oldest specimens have been reared for 2 weeks. None of these larvae 

 has reached the stage of development shown by Mortensen's plutei 

 obtained from plankton samples from the West Indies and from the 

 Indian Ocean. In my oldest or most advanced larvae (fig. 56) the 

 fenestrated rods have increased noticeably in length and the antero- 

 lateral rods have extended well forward into the anterior end of the 

 larva (fig. 5a). No contact between the ventral transverse rods of 

 opposite sides or of body-rods has yet occurred. 



Formation of Mesenchyme in Echinoderms. 



The manner of formation of mesenchyme that I have described 

 is similar to that occurring in Antedon as described by Seeliger, and 

 which has been regarded as typical of Crinoids. Mortensen (1920), 

 in his study of the development of the Crinoid Tropiometra carinata, 

 has discovered that in this form, prior to the invagination of the 

 wall of the blastula in the formation of the endoderm, there is a migra- 

 tion of cells into the cavity of the blastosphere, probably from dif- 

 ferent places in the wall of the blastosphere; ''these cells lie loosely in 

 the cavity and look like mesenchyme cells, which, however, they are 

 not" (p. 8). When the cavity of the blastula is nearly full of these 

 cells the typical invagination begins and the loose cells become closely 

 applied to the upper end of the invagination. This process is in 

 striking contrast to the condition in Antedon, in which Seeliger found 

 no free cells in the blastocoele until after the invagination of the endo- 

 derm. Mortensen (1920), in his study of the development of one 

 of the viviparous Crinoids, Isometra vivipara, found a cleavage of 

 the kind typical of Arthropods; there is a division and migration of 

 nuclei before any cell-walls appear; ectoderm and endoderm are 

 differentiated in place, no invagination occurring. There are no 

 cells in the space between ectoderm and endoderm, but some yolk 

 grains may be seen lying in this cavity. 



Guthrie and Hibbard (1919) have given a summary of the facts 

 known regarding the time of formation of mesenchyme in the various 



