XVI 



ECHINODEEMATA 



527 



motionless larvae which show by their clear protoplasm that they 

 are healthy. These larvae are divided by a constriction into two 

 sections, one of which shows by the nature of its cells that it 

 corresponds to an inverted gut, and the other corresponds to the 

 ectodermic skin of the gastrula. The effect, then, of the lithium 

 would be to alter the conditions of pressure in the blastocoele so 

 that the gut develops outwards and not inwards. 



If, as we have seen reason to believe, the process of gut formation 

 is analysable into two factors, viz. cell multiplication and invjardly 

 directed cyfotaxis, then the lithium has reversed the direction of the 



Pig. 393.— Types of lithium-larva. (After Herbst. 



A, blastala stage of lithium-larva of SphaerecMnus granularis ; ectoderm and endoderm are not yet 

 differentiated. B, later stage in the development of the same larva; the ectoderm carries the long cilia. 

 C, still later stage in the development of the same larva ; the ectoderm is beginning to be grooved off 

 from the endoderm. D, stiU later stage ; the groove is deep, and the endodermic portion exhibits a 

 secondary division into a small upper part representing the intestine (?), and a lower part representing 

 the stomach of a normal larva. E, F, G, lithium-larvae of Spluierechinus granula/ris, in which the lithium 

 has acted more intensely, in B the ectodermic portion is very small, in F it is reduced to a mere knob, 

 in G it has disappeared, ect, ectodermic part of larva. 



cytotaxis. But that is not all. The longer the eggs lie in the 

 mixture of sea-water and lithium solution, and the more of the 

 lithium solution used, the higger is the portion of the larva which 

 corresponds to the gut. This can go so far that practically the 

 whole of the larva becomes gut, the ectoderm being represented by 

 a little knob of cells at one end of the vesicle. 



Herbst suggests that when the blastula stage is reached the 

 physiological separation of ectoderm-forming and endoderm-forming 

 substances is hurried on. He supposes that, in a normal blastula, 

 the amount of endoderm-forming substance increases as we proceed 



