106 HIROSHI OHSHIMA 



Cases of situs inversus viscerum are not very rare in nature, 

 and arc frequently met with under artificial conditions. 

 Spemann (29, pp. 400-14), in his most interesting experi- 

 mental studios on Triton larvae, has made an exhaustive 

 survey on cases of situs inversus. According to him (p. 401) 

 the cases may bo classified into two categories, though the 

 distinction between these two may not be clean-cut. The 

 one comprises such cases where an ' inverting ' factor affects 

 an individual very early in its ontogeny, it may be even before 

 fertilization, so that the ' microstructuro ' of the egg under- 

 goes a change at once and completely. Those Gasteropods 

 with reversed spiral belong to this category. Conklin 

 (3, p. 585) suggested as its cause the reversal of the polarity 

 in the egg. 



To the second Ijelong those cases where that factor acts much 

 later in the embryonic development, a little while previous to 

 the time when any visible asymmetry of organization occurs. 

 It affects only a single but decisive part, and in consequence of 

 the abnormal development of that part all the other adjoining 

 organs will assume the inverse situs. There are many interest- 

 ing instances of this : thus, for example, a chick embryo 

 heated on its left side (Dareste, Warynski and Fol), 

 a Triton embryo with a portion of the medullary plate 

 cut out and replaced in the inverted position (Spemann), 

 an egg or embryo which has been constricted along its median 

 plane partially or completely so as to give rise to either a double 

 monster or twins (Spemann; compare Bates on, 2, 

 p. 560, and Morrill, 18, p. 267), and two halves of embryos 

 with different rate of growth grafted together (Spemann) 

 can likewise produce the situs inversus. Cases of such partial 

 situs inversus have also been interpreted in a most satisfactory 

 manner, as has also the striking fact that generally the abnor- 

 mality is exhibited by the right-hand members of double 

 monsters of Triton (and trout) and by the right-hand 

 member of twin Triton larvae. 



Turning now to the case of the reversed Echinus larvae, 

 I have tried to propose tentatively an interpretation. This 



