428 HOLMES. [Vol. XVI. 



arranged in an asymmetrical but perfectly definite manner ; 

 but Zur Strassen found that in exceptional cases — in one egg 

 out of about forty — this asymmetry was reversed ; that is, the 

 arrangement of cells, normally occurring on one side of the 

 embryo, was found on the opposite side, one embryo being 

 the mirrored image of the other. As the adult Ascaris is, in 

 certain respects, asymmetrical, Zur Strassen was led to ascer- 

 tain the proportion of reversed specimens in this form. It was 

 found that, out of 125 individuals, four were reversed. Reversed 

 adults occur, therefore, in about the same proportion as reversed 

 eggs. As reversed eggs were seen, even in advanced stages of 

 development, in an apparently normal condition, Zur Strassen 

 concludes that they develop into reversed adults. We have 

 here a reversal of cleavage occurring as an exceptional varia- 

 tion in eggs of the same species and probably giving rise to a 

 reversal of some features of the structure of the adult form. 



In Planorbis I have been unable to trace the origin of asym- 

 metry to the cleavage of a single entodermic cell, as Conklin 

 did in Crepidula. Immediately before gastrulation begins the 

 entoderm is composed of a nearly circular patch of small ento- 

 meres, which are quite numerous (over 30), and of nearly equal 

 size. I am quite certain that, before invagination, the entoderm 

 gives no hint of the direction of the coil of the adult animal, 

 nor does it manifest any appreciable asymmetry until a long 

 time after the gastrula stage. In the gastrula shown in PI. 

 XX, Fig. 46, the bilateral symmetry is almost perfect. This 

 gastrula contains several hundred cells ; yet, if one carefully 

 examines the figure, which is an exact camera drawing showing 

 the outline of every cell, it will be found that for nearly every 

 cell on one side of the body a corresponding cell can be found 

 on the other side. The only deviation from bilateral symmetry 

 that could be observed in this gastrula was a slight torsion in 

 the cells of the head vesicle. The torsion can be observed in 

 most gastrulae, but whether it has any connection with the final 

 asymmetry of the animal could not be ascertained. Attention 

 has been called to the fact that the arms of the cross exhibit a 

 slight twist in a laeotropic direction, and that the direction of 

 the twist is doubtless connected with the reversed cleavage of 



