EXOGASTRULATION AND RELATED MODIFICATIONS. 



6 9 



lay down an excess number of skeletal spicules (3, 4, or 5 the 

 total final number in ordinary development or even 7, in 

 Sphcerechinus) and the pluteus arms are increased in number and 

 displaced accordingly and had besides an increased number of 

 radii '4 or ,S in-trad of 3). 



\ joo I. id >lows down development noticeably, but the 

 bla-iul.i- are of nearly normal size and less crowded with mesen- 

 ch\mc, wliich is extruded from the posterior end in only about 

 half of the embryos. When gastrulation occurs there are found 

 about 60-70 per cent, entogastrulae and only 30-40 per rent. 

 exogastnilae and intermediate forms. The bla>tula iv \ n \\\^ 

 in >ea water quickly resume active movements and I>L--..I 

 trul. i- differing from the lithium larvae in the slightly thinner 

 drrni and entoderm; in the incomplete or merely tem|orary 



3 g- j- ftl ' t>) evagination of the gut; in the relatively 1 



a 



e 



J 



FIG. 5. Sand-dollar embryos as moditic-l l.\ < 50 i 1 . . MK! dilute 



. .ii IT. a. an -at 1 trula after i day in CuSO, M 1,250 . alter 2 days 



it-nil II . < ,M .ooo.ooo; c-. I'ri'in diliiti A.UIT; j-m. from 

 K\i \l 750,000. 



