428 CASWELL GRAVE 



Yolk segregation in other echinoderms. The eggs of practi- 

 cally all species of echinoderms are niinute bodies and the small 

 amount of yolk material they contain seems to interfere but 

 slightly if at all with either the activities of the cells of the devel- 

 oping larvae or with other processes of development and dif- 

 ferentiation. In all cases observed the yolk material of echino- 

 derm eggs is uniformly distributed throughout the cytoplasm 

 and retains the same uniform distribution within all cells result- 

 ing from segmentation of the egg until it is finally absorbed 

 during larval development. 



Ophiura brevispina (and probably all species of the genus 

 Ophiura) and a comparatively few species scattered here and 

 there among various genera of all classes of echinoderms (Asterina, 

 Cribrella, Solaster, Thyone, Antedon, etc.) differ from the more 

 typical species of the phylum in that they produce eggs of rela- 

 tively large size, containing a rich supply of yolk material. 



The distribution and fate of the yolk material during the 

 development of each of these species merits more careful study 

 than it has yet received, and it is not a subject concerning which 

 generalizations can safely be made. A comparison of the 

 observations that have been recorded by Masterman ('02) on 

 Cribrella and by Gemmill ('12) on Solaster with those of this 

 paper, indicate that some sort of yolk segregation process may 

 be expected to take place in the development of all yolky eggs 

 of echinoderms but that each species has independently developed 

 a segregation process pecuhar to itself and without genetic 

 relationship to that of other species. 



Cribrella. In Cribrella, according to Masterman, both the 

 segmentation and archenteric cavities of the larva become filled, 

 soon after gastrulation, with cells which he designates mesen- 

 chyme and hypenchyme respectively. Yolk globules of vary- 

 ing size and shape are shown by Masterman (figures 26, 27 and 

 28) in all cells of the late gastrula; epiblast, hj^oblast, mesen- 

 chyme and hypenchyme, but it does not appear from either 

 figures or text that yolk material is present in relatively greater 

 quantity in the mesenchyme or hypenchyme cells than in the 



