334 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



24-hour Stage, the susceptibihty conditions are reversed and mega- 

 cephaly resuhs (fig. 154). 



It is interesting to find that in the microcephahc forms the an- 

 terior trunk region is absolutely larger than in controls, while the 

 same is true for the heads of the megacephalic forms. This is to be 

 explained very simply. There is a definite limited quantity of food 

 material available in the egg; and when one region is inhibited, 

 regions which are less aflfected are able to obtain a greater share. It 

 is not known how long the modifications of proportion thus ob- 

 tained will persist, although, in Arenicola, forms with some degree 

 of posterior inhibition have been reared through metamorphosis. 



Other interesting experiments have been carried out on sea- 

 urchin eggs.^ Exposure to inhibiting concentrations of KCN 

 throughout early development results in plutei in which apical 

 regions, notably the oral lobe, are relatively under-developed. The 

 posterior (basal) regions are therefore relatively over-developed, 

 and consequently after such treatments narrow-angled forms are 

 produced, in which the arm spicules may even be parallel. 



In weaker solutions, where differential acclimatisation can occur, 

 the reverse process is found. The oral lobe is relatively enlarged, 

 and the skeletal arms diverge at a wide angle. In extreme cases, 

 types of highly abnormal proportions are produced (fig. 155). 



The well-known "lithium-larvae " of Echinoderms may be men- 

 tioned in this connexion. It was early discovered^ that when sea- 

 urchin eggs are reared in a medium to which lithium salts have been 

 added, forms known as exogastrulae are produced, in which the 

 archenteron is present, but evaginated instead of invaginated 



(fig- 156). 



Exogastrulation is the most obvious eflFect of this treatment, but 

 it appears to be a secondary result. The primary effect of lithium is 

 to decrease the amount of ectoderm produced by the egg, and to 

 increase the amount of endoderm, progressively with increasing 

 concentration. The skeletogenous cells move towards the animal 

 pole. Exogastrulation is probably a mechanical effect, the de- 

 creased ectodermal area being unable to accommodate the enlarged 

 archenteron in its interior. If the lithium treatment is continued 

 until the middle blastula stage, " holoentoblastulae " may be 

 1 Child, 1 91 6 b. 2 Herbst, 1895. 



