528 INVEETEBEATA chap. 



from the animal to the vegetable pole. When the endoderm-formiug 

 substance accumulates, absorptive power is increased and more 

 lithium is taken in. The effect of this is to increase still further 

 the amount of endodermic substance and inhibit the formation of 

 ectodermic substance, and so more and more of the blastula wall 

 is transformed into endoderm. 



When larvae in which there is still some ectoderm are trans- 

 ferred to sea-water, they acquire the power of ciliary movement, and 

 develop an abnormally large number of calcareous spicules which are 

 arranged in a circle, and a correspondingly large number of arms is 

 developed ; so that the formation of an arm is dependent on the 

 stimulus afforded by the presence of a spicule. 



We ourselves (1911) have found that in the larvae of both 

 Uchinus miliaris and of Uchinus esculenius a right hydrocoele is 

 occasionally developed. In such cases a right amniotic cavity may 

 be developed, on the floor of which typical pointed spines may arise. 

 Further, from the right posterior coelom a series of dental sacs or 

 perihaemal rudiments may be formed, and finally a second " adult " 

 oesophagus and mouth. This shows that neither the normal 

 formation of an amniotic invagination from the ectoderm of the 

 left side, nor of dental sacs from the left posterior coelom, nor 

 of an adult oesophagus from the left side of the stomach, is due 

 to the pre-existence of an invisible rudiment of the structure to be 

 found in the layer of cells out of which it is formed ; on the con- 

 trary it is clear all three developmental processes mentioned must be 

 due to an influence from the hydrocoele acting on the indifferent 

 sheets of tissue constituted by these layers, i.e. on ectoderm, coelomic 

 wall, and stomach endoderm respectively. Such influences are termed 

 formative stimuli. 



Hence we may make to ourselves the following provisional 

 picture of Echinoid development. Very early, before cell division 

 has occurred, there are constituted in the cytoplasm, by the influence 

 of the nucleus, definite substances which can cause the formation 

 of the primary organs. The formation of the substances cannot 

 be attributed to the daughters of the primary nucleus, because then 

 it would be impossible to disarrange these daughter nuclei and 

 yet obtain a typical result. 



These substances are at first uniformly distributed throughout 

 the egg ; as segmentation goes on they become segregated from one 

 another, and when this segregation is complete the formation of the 

 layers, that is, of the primary organs, is begun. When once these 

 layers are formed they in turn produce substances which act on each 

 other and cause progressive differentiation. These substances can be 

 increased or decreased by the action of certain salts. So long as 

 a fragment of an egg has a certain minimal proportion of an organ- 

 forming substance, the organ will be formed in typical develop- 

 ment. 



Driesch's pons asinorum regarding the formation of a gut of small 



