EXOGASTRULATION AND RELATED MODIFIt \llo\-. 



the vital fundamental process; the direction towards \vhirh the 

 \vth pro< eed- in exogastrulation is 'determined by the sur- 

 roundi; It i- -u restive that both these leading experimen- 



tali-t- appealed to -ome more or less differential growth pro. 

 Inn in neitlu s the evidence more than meager, indirect, 



and i-olated. 



An..' k'-d t'roin the new viewpoint of differential inhibition, 

 en .. lithium ariion is seen to be interpretable in a \vay not only 

 entirely eon-i-ietit and coordinate with a vast body of modern 

 \\ork in general and special physiology and of physiological form 

 deiermination and teratomorphy, but also substantiated by 

 < on~iderable direct proof and many suggestive parallel- (.p. 78. 

 and Child, '20). Since Li' in the concentrations actually m 

 -ary is certainly inhibitory and since the inhibitory ellei '- arc 

 felt rhietly at t lit- animal pole and in its differential i< m- and Ica-t 

 in the mesenchyme and the entoderm, it is unlikely that t la- 

 Li' act- >o much directly on the basal regions; it may be said 

 i.it her to release these regions from the control of tin- ectoderm 

 b\ checking the (usually dominating) activity of the hitter. 



I determine the more proximate causes of the inhibitory 

 effects will require much further detailed observation and e\- 

 periment (for each separate agent) on sites and rates o! penetra- 

 tion, water imbibition and withdrawal, alteration ot -in: 

 ten-ion and chcmotaxis (Rhumbler, '02), precipitation-. Dela- 

 tion-, -olmions, etc. Into whatever ultimate cau-al compoiieni- 

 resolved the end rc-ull remains, physiolou' 11\ . .1 dittereni ial 

 inhibition. 



I -irulatioii is evidentK' a secondarx manifestation o| thi- 



di Herein ial action alonj; a le\'elled or iv\ er-ed metabolic gradient . 

 Since the lithium-modified embrvo i~ merely a len;<l heiied and 

 differentiated e^ in \\hich endogastrulation ha- been |re\i-nied 

 Fij ii ' -.' v .-irulation mii^ht be con-ii leici I t \ en a direi t 

 re-nlt of the inhibition of the-i le\elliir^ the normal -radit-m 



nece>sar\ \< >v en< h \gas\ rulai ii ui . 



But a mechanic.il factor -eein- lo be involved. K\paii-ion and 

 o\er^rowth of the ectodcrmal i'(Mlhelial la\'er i- checked b\ the 

 tailure of the u-nal multiplication and enlargement of ii-, cell-, 

 whik- growth and cell di\i-ion of eiitodcnn are le--. and tho-e of 

 mc-ciich\me least retarded. Consequently the ecto.lcrm.il cell- 



