OF THE GERMINAL LAYERS TN rjARTROPODA. 31 



tlie (Iniwing^ in his Fig. o (illustrating the vegetative pole of the 

 egg) which is like my Fig. -57, is interpreted in a different \\<\y. 

 The hilateral cell, to which I have called attention, is in our species 

 pi'odnced at nhout this time from the posterior half r.. and d,. of 

 the fourth generation of ectomere-cells. Such a cleavage really 

 occur also in A. limacma ; but it is said to be accompanied or- 

 dinarily with the simultaneous division in its anterior components. 

 Hence, in that species the new generation is formed of a quartet 

 like all the preceding ectomeres and does not influence in any 

 way the transformation of the cell-arrangement. Nevertheless 

 the bilateral form of cleavage occurs also in the ectomere-cells 

 o6'^ and of?\ as may be gathered from the author's own words as 

 "subito dopo, alia 28" ora oc^ e 3(/^ si dividond con fusi transversali, 

 cioè con divisione bilaterale, mentre oa} e ?)b^ rimango in riposo." 

 However, this is the second division of those ectomeres, and indeed 

 after the l)ilateral symmetry is fairly established by the formation 

 of the mesomere-cell. Thus the transformation of the body sym- 

 metry here seems to be effected l^y the gradual and renewed dis- 

 2:)Osition of already formed ectomeres rather than by a single 

 cleavage of the posterio]* half of the fourth ectomei-es. Such a 

 method of transformation is seen in various species as T have 

 already noted. 



Concerning Georgewitcii's paper on A. depUans the author, 

 it appears to me, has fallen into some confusion on important 

 points. At all events his ideas on the orientation of the egg are 

 exastly contrary to those usually held, tlie portion called by him 

 the posterior, being really the anterior, so that his " Ur- 

 mesodermzell " is in fact derived fi'om the anterior blastomere. 

 And it is very strange that he describes the posterior (really 

 anterior) half of the second quartet of ectomeres as the " Ur- 



