EXOGASTRULATION AND RELATED MODIFICATION-. 75 



its chief apico-bas.il axis at a glance. Here the manifestations 

 of olc.it li are tir-t visible at the animal pole or apex and pn>^: 



dually and -ic.idily towards the basal region which is reached 

 l,i~i, often hour- .tfter the opposite end had been affected. The 

 apparent n-ver-e in some slow-acting solutions was obviously 

 due eiiiin-ly to the basal extrusion of the mesenchyme. 



Tin- same gradient persists into the gastrula stage, where the 

 apex -hows the greatest susceptibility, while the inva^inated 

 MI., derm is so much more resistant (except probably for a /one 

 around the blastopore) that it usually survives in M [OO KNC an 

 hour or more after the ectoderm of the entire gastrular \\all has 

 disintegrated. By returning non-disintegrating gastrula 1 to 

 sea water after an appropriate exposure the ensuing dish ion 



may be checked midway in its course and recovery I.ir\.r 

 ol it, lined showing absence or partial loss of apical (or even nioir 

 basal) ectoderm and more or less complete survival of the ento- 

 dermal parts which are accordingly large in proportion to the 

 whole. As in the case of Arbacia "in the gastrula the apex of the 

 conical both' represents the apical end of the major axis, and the 

 Mastopore the basal end, while the entoderm represents a still 

 more basal region of the egg and blastula, and the mex in livme 

 the most basal region" (Child, 'ib/>, pp. 68-69). 



Vital staining with neutral red and other basic dyes shows an 

 apico-ba-al progression in blastul.e and early gastruhe quite 

 paralleling in all essentials the wave of advancing disintegration 

 as de-cribed above. 



The experimental embryologists have described the i-oinni"n 



a d c 



FlG. ii. Diagram illu-tratiiiK tin p..|.ir ;nr.m.o nn lit ..I part- in tl 

 the normal i-inlu iiicl in litliiinu i-mlii-. .iiinili-iin 



mill-, and hind-gut. The lithium rml>ryr> i~ ii-.i!l\ an <-!i iny.iti <\ .ni'l 



with the germ-' 

 tlu- . 



