554 KNOWER. [Vol. XVI. 



segments or the anus, the comparison with the embryonic, 

 invaginating rudiment of the apterygota and myriopod should 

 be made with the ectoderm alone, and at a correspondingly 

 early stage in its differentiation. 



My effort is not to derive the amfiion from a portion of the 

 myriopod body in a phylogenetic sense ; but to explain how, in 

 association with a fold similar to that of the viyriopod-like 

 ancestor, but appearing sooner, it may represent aji early special- 

 ization of the undifferentiated tissue {ectoderm) of a primary 

 embryonic rudiment common to the two arthropods {see B, pp. JO- 

 JI); and how this folding off of the amnion need not prevent the 

 usual continuatioji of the development into an elongated embryo 

 comparable to the myriopod. 



Such a comparison may be readily made by referring back to 

 the diagrammatic text-figures. 



These figures of course represent actual stages in the devel- 

 opment of the three forms. The two upper figures illustrate 

 the first appearance of the ventral flexure (doubling-up) of 

 a myriopod, Text-fig. i (Julus, after Heathcote), and of a 

 wingless insect (the apterygote Lepisma, after Heymons), 

 Text-fig. 2. 



Following Heathcote (31) in his description of the myriopod 

 development, we find in the first text-figure that the ventral 

 flexure occurs here comparatively late in the ontogeny, after a 

 few anterior segments have been formed from both layers. The 

 important fact to note is that the bending takes place just 

 behind the last segment differentiated, and in a region that 

 Heathcote speaks of as unspecialized tissue, commonly termed 

 the "tail-piece." I have indicated in the diagrams the usual 

 sharp distinction between the early ectoderm and mesoderm in 

 this posterior region, Text-fig. i (see Heathcote's Fig. 30). 

 (Note that the as yet undifferentiated ectoderm is striped in 

 the diagram, while the similar mesoderm is a simple black line.) 



Examining Heymons's results for Lepisma, as represented 

 in the second text-figure (Text-fig. 2) to the right, we find 

 essentially the same conditions as in the myriopod. (See his 

 Fig. I, (15), for the sharp separation of primary ectoderm from 

 mesoderm.) 



