KINETIC AND CAUSATIVE ASPECTS 



343 



(c) The localisation of the blastopore in Anamniots. An operative approach 



This gives value to my earlier suggestions of 1935a and to the experiments which have been 

 done on the eggs of frog, newt, axolotl, trout, minnow, gold-fish, pike and others. Again, for 

 historical reasons, amphibians have to be considered first. For more than half a century, it has 

 been known that if frog (or newt) eggs are put upside down when still undivided or while 

 at the 2- or even 4-cell stage, and are maintained in that reversed position until cleavage 

 is fairly advanced, gastrulation will be deeply affected (Fig. 30, 31). From the analysis 

 of innumerable results, two relations have come to light. (/) A blastoporal groove will 

 appear (Penners and Schleip, 1928; Penners, 1929; Pasteels, 1938) at any place where a 

 packet of big yolk platelets have been incorporated in the egg cortex; around these few 

 large yolk cells, an intermediate brownish ring (frog) of less loaded cells will be formed, 

 and at least part of this ring will invaginate with the customary aspects of bottle-necked 

 cells. {2) A yolk packet is not the only prerequisite for gastrulation. When the mass of 

 deutoplasm is a bit large, it can be observed that gastrulation prevails on one side; 

 there, morphochoresis begins earlier and more cells invaginate and stretch than on the 

 other side. If an embryo can be formed, its somites, pronephros and even its brain and spinal 

 cord are more strongly built on the favored side. The difference has been demonstrated 

 to be caused by the previous location of the grey (or light) crescent (Fig. 32), and it could 

 be established that the whole cortex may be considered as a gradient field, with activity 

 decreasing from the grey crescent in all directions. The observed asymmetry indicates 

 that at the border of a transposed or reduced yolk mass, a kind of activating reaction takes 

 place between yolk derivates and the cortex (Dalcq and Pasteels, 1937, 1938). These two 

 notions have been the core of a theory that in a normal egg a similar interaction is respon- 

 sible for the starting of morphochoresis. The resultant, in a given territory, of this inter- 

 action is meant to endow this area with a given morphogenetic potential, an expression also 

 used, in the same period, by Yamada (1940, his p. 195). In a fertilized egg which has 



gr.c 







iOI 



d fy 



Fig. 30. Experiments of oriented rotation (a, b) and of complete inversion (c, d, e) 

 on the fertilized frogg egg. (a) The egg is oriented on a slide, to which it adheres by its 

 jelly, in such a way that the vegetal pole (Vg) is uppermost and a little to one side (An = 

 animal pole). The egg is free to rotate within its vitelline membrane and the greater 

 density of the yolky vegetal pole causes it to move in the direction shown by the arrow. The 

 meridian of rotation will become the plane of bilateral symmetry (b) and the grey crescent 

 (gr. c.) will appear {of. p. 324 for explanation) on the side where the vegetative pole was 

 originally placed, (c-e) Technique for keeping a fertilized frog's egg in a fixed inverted 

 position (O. Schultze). The egg, on the point of dividing, is placed between two slides, 

 kept a certain distance apart by small pieces of modelling wax (W) ; (c) general view of the 

 arrangement; (d) vertical section before reversal; the pressure distorts the perivitelline 

 space, and prevents the egg from returning to its normal position; (e) the same after 

 reversal; only the yolk (y) is affected by gravity (indicated by arrows). From Dalcq, 1952. 



Literature p. 483 



