14 Hybridization of Echinoids. 



throughout the blastocoele. In plate 3, figures a to e, sections of the 

 posterior portion of embryos in process of gastrulation are shown. In 

 plate 3, figure a, cells are in process of migration from the wall; in plate 

 3, figure b, mesenchyme cells are at the inner end of the archenteron 

 and at its base. Figures c, d, and e of plate 3 are from the same 

 slide and are all of embryos 24 hours old. They have been selected 

 because they show considerable individual variation. In figure c of 

 plate 3 no cells have left the wall of the archenteron; in figure d of 

 plate 3, mesenchyme cells may be seen at the base of the archenteron ; 

 and in figure e of plate 3, mesenchyme cells are to be seen at the base 

 of the archenteron and in process of withdrawal from its wall. It 

 is of interest to compare these sections of the hybrids, age for age, 

 with the sections of the normal larvse. The corresponding ages are 

 figure A of plate 3 with f and g, all 18-hour embryos; figure b with h, 

 both 23-hour embryos; and figures c, d, and e with i, all 24-hour 

 embryos. 



Succeeding stages show considerable variation. In some the 

 growth of the archenteron ceases and the blastocoele becomes filled 

 with a mass of opaque cells (fig.Se). In others gastrulation continues 

 slightly beyond the stage indicated in figure e of plate 3, In a few 

 cases a small triradiate spicule was found in the mass of mesenchyme 

 cells at the right and left of the base of the archenteron. I was 

 unable to keep the hybrid material alive beyond the gastrula stage. 

 No plutei whatever were obtained. 



It is at this stage of beginning gastrulation that many attempted 

 hybridizations between species of Echinoids fail. The material from 

 which sections represented in figures b to e of plate 3 were made 

 enables us to see clearly that the failure in development is accom- 

 panied by an extreme degeneration of nuclei. In figure b the cells 

 above the inner end of the archenteron seem to be in a process of 

 disgorgement of chromatin; in the same figure a mesenchyme cell 

 on the lower left side is shown which has completed such a process. 

 In figure c two of the cells in the wall at the lower right are degener- 

 ating, while in figures d and e numerous stages in karyolysis are 

 shown; the nucleus in some instances seems to have disintegrated, 

 in others to have become simply a deeply stained mass of chromatin 

 which may even be extruded from the cell. In some sections these 

 distinct masses of deeply stained material may be found in consider- 

 able abundance among the scattered cells. 



When we review this series of changes in the development of the 

 hybrids and compare this development with the development of 

 Cidaris and with the corresponding blastula and gastrula stages of 

 Lytechinus (figs. 3, b, c, and d) and of Tripneustes, it may be seen 

 readily that the processes of development in the hybrid are inter- 

 mediate in character, lying between those of Cidaris and those of 



