EXOGASTRULATION AND RELATED MODIFICATION- 77 



respectively. Obviously in the lithium-modified embryo tl it- 

 egg ha- merely elongated and differentiated without tin- usual 

 infoldii 



2. Lithium Embryos. Tested with the same agents in prc- 

 cisely tin -aim- way as were the normal embryos the exogastrular 

 .UK! con>triction forms (p. 63) showed new and different and 

 charactc ri-tir susceptibility relations. General susceptibility is 

 much less than in the normal, and the original apico-basal 

 gradients of susceptibility, vital staining and decolori/atiim are 

 less marked, absent or reversed (Fig. 12). 



Quite commonly the basal end of the entoderm ha> already 

 in disintegration in the LiCl solution itself, and much of tin- 

 second a r\' mesenchyme may have been removed and tlu- primary 

 nchyme may have escaped from the blastocoel (Fig 12, /, /). 



Newly formed lithium exogastruhe and constricted forms 

 removed intact to M,. 1000 KNV, M 500000 HgCl-_>, or concen- 

 trated neutral red. disintegrate in the next few hours. In this 

 process disintegration and decolori/ation after staining generally 

 I Jus in the secondary mesenchyme-forming region of the an h- 

 enteron. 



Sometimes the two extreme ends disorganize at the same time, 

 the apical ectoderm of the gastrula wall being as suscrpiibile as 

 tlu- basal entoderm of the archenteron. And in a few cases the 

 ectoderm goes a little in advance of the opposite end. In these 

 hour glass forms the susceptibility varies with the size of tin- 

 components; the larger component being quite uniformly tin- 

 more susceptible; and indeed it is apparently susceptible in din < t 

 1 'i' 'I ioi lion to its relative size, as the sketches str^e-t. I 

 greater the reduction of the ectoderm the higher i- it- resistai 



Exogastruiac removed from the lithium in a more differentiated 

 state with bi- or tri-partite gut, etc., exhibit regularly the re- 

 versed order of susceptibility of parts I 



DISCUSSION AND IMI-RI-KI i \ i h >\. 



i. Physiological Gradients in I-'.ihiniil /-'.: hryos. 



It will be taken as highly probable that in the e.^g and blastula 

 of the echinoderms so far studied the >u-ceptibility declines from 

 the apical animal pole region to the ba-al \e-etal pole levels. In 



