1904.J PACKARD — OPISTHENOGENESIS. 293 



Prof. E. B. Wilson^ has clearly stated the nature, now so well 

 known, of the growth-processes involved in the interpolation at the 

 growing point or budding-zone of new segments. In Polygordius, 

 after the trochosphere has been formed and when it is about to 

 enter on the adult stages, the segments are formed successively, 

 those in front being the oldest, '* while new segments are contin- 

 ually in process of formation, one after another, at the growing 

 point." This, he says, is ''a typical case of apical or unipolar 

 growth." It is what we would call opisthenogenetic growth. 



Professor Whitman^ has shown that in the leech the internal 

 tissues (mesoblast) of the budding zone are arranged in two widely 

 separated lateral bands which, to quote Wilson's exposition, '*as 

 the trunk grows older, widen out and grow together along the me- 

 dian line, ultimately giving rise to muscles, blood-vessels, excretory 

 organs, reproductive organs, etc." Now if this is the case with the 

 more important tissues, why in caterpillars as well as in lizards may 

 not this opisthenogenetic mode of growth also involve the arrange- 

 ment and distribution of the pigment-masses of the integument ? 



Without entering into the mode of development of the germ- 

 bands, which are behind completely separated, gradually becoming 

 united in front, resulting in their union or concrescence, we would 

 make the suggestion that this phenomenon may be the initial cause 

 or at least in some way connected with the breaking up of the lon- 

 gitudinal stripes of the body, and their transformation into spots at 

 or near the budding zone of their polymerous or polypodous (Peri- 

 patus-like) ancestors. 



In the trilobites, Limulus and Diplopods, the new segments after 

 embryonic life are interpolated between the penultimate and anal 

 or last segment of the body, and it is from this region in certain Lepi- 

 dopterous larvae that the transformation of longitudinal stripes into 

 spots takes place. The question next arises whether there is any 

 connection between the opisthenogenetic origin of the markings of 

 lizards and that of caterpillars. The fact, now well established by 

 embryologists, that the phenomena of concrescence occurs not only 



"^ Some Problems of Annelid Morphology. Biologicil lectures delivered at 

 the Marine Biological Laboratory at Woods Holl, 1891, p. 61. See also A. D. 

 Mead, " The Early Development of Marine Annelids," Jourrtal of Morphology ^ 

 XIII, May, 1897, PP- 227-326. 



2" The Embryology of Clepsine," Joiirn. Micr. Se., XVIII, 1878; Journal 

 of Morphology, Boston, 1887. I am indebted to Prof. A. D, Mead for calling my 

 attention to the concrescence process in this connection. 



