SITUS INVERSUS IN ECHINOIDS 135 



instances where an ectodermal invagination was formed at 

 a place under which no hydrocoele had been developed 

 (Cases B, 22, 23, and also 25, p. 271), He further made experi- 

 ments to prove his view that the formation of the amniotic 

 invagination is a self-differentiation and is not formed from 

 stimulus of an underlying hydrocoele. He could produce a new 

 amniotic invagination in a larva of Echinus m i 1 i a r i s 

 from which the echinus-rudiment had been removed (27, 

 pp. 9-11). In another of his experiments an amniotic invagina- 

 tion was seen to appear in each of two pieces of a larva where 

 the normally-formed invagination had not been included ; 

 thus in this larva three amniotic invaginations in all were 

 formed (pp. 13-14). It may be mentioned that in all of his 

 cases the ectodermal invagination was very small and lined 

 with flat epithelial cells. In another place he states (25, 

 p. 302) that the invaginated ectoderm forms cylindrical cells 

 only at the place where the hydrocoele wall comes to be in 

 contact, while in the other part the cells remain flat. I myself 

 understand by the term amniotic or ' echinid ' invagination an 

 ectodermal pit whose epithelial cells are from its first appearance 

 high and cylindrical, even when fairly apart from the hydrocoele 

 (Text-figs. 2, D, and 3, c, am). In this sense I cannot help 

 doubting whether all of Runnstrom's structures deserve 

 the name amniotic invaginations. He admits that the further 

 development and differentiation of the amniotic invagination 

 is conditioned by the presence of a hydrocoele, and that 

 without it the former degenerates (25, p. 305). It is of interest 

 to see that he pointed out that the role of an amniotic invagina- 

 tion could be played, to a less extent, by other ectodermal 

 invaginations, such as that which he termed ' spine invagina- 

 tion ' (Case B). According to him, if there was no amniotic 

 invagination formed the stone-canal stopped developing when 

 it had reached its normal length, and later gradual degenera- 

 tion set in of the whole water-vascular system. But, as for 

 the larva in question, the ' spine invaginations ' were situated 

 further back than normally an amniotic invagination is 

 placed, and the stone-canal did not stop at the normal length. 



