292 PACKARD — OPISTHENOGENESIS. , [June 15, 



of such fusion have appeared in the pair on any of the segments in 

 front. 



When the ontogeny of Nerice bidentata is worked out, it will be 

 a matter of much interest to observe whether the dorsal humps are 

 formed from behind forward, or whether they appear simultaneously, 

 and thus form an apparent exception to the law of transfer of 

 growth-force from behind forwards. 



In this connection it might be observed that in the larva of 

 Schizura unicornis, in which there is the very unusual occurrence of 

 a pair of short thick spines on the vertex of the head (PL XXVIII, 

 Fig. 2, 2a, 2b), these spines do not appear in stage I and not until 

 after the first molt. These spines persist through stages II and III, 

 but after this disappear, not being present in the two last stages. 

 Thus the growth-force resulting in the development of the armature 

 of stage I does not reach the head until after the first molt, and 

 then does not persist throughout larval life. 



In the ontogeny of the Notodontian family, as well as that of 

 Ceratocampidae and Saturniidae, the process of fusion of the two 

 dorsal tubercles always first begins on the eighth abdominal 

 segment. 



Opisthenogenesis, as regards the markings, appears to be of a 

 piece, or somehow connected, with the opisthenogenetic origin in 

 post embryonic development of new segments. In the cestodes 

 and in annelid worms, multiplication of segments occurs between 

 the head-region and the extreme end of the body. Thus in Poly- 

 gordius, as stated by Balfour {A Treatise on Comparative Embry- 

 ology, i88o, I, pp. 271, 272), the conversion of the larva into the 

 adult takes place '•' by the intercalation of a segmented region 

 between a large mouth-bearing portion of the primitive body and a 

 small anus-bearing portion." 



This region in the larval or early stages of worms and more 

 primitive arthropods is the "budding zone" of embryologists. 

 While at the outset, in the beginning of embryonic life, the head - 

 region is the first to be formed and the trunk-segments arise later, 

 as in the trochosphere of worms and the protaspis of trilobites and of 

 merostomes, a third portion, arising from the budding zone or seat 

 of rapid cell-formation, appears to be a secondary or inherited 

 region, due to the post-embryonic acquisition of new characters 

 (certain trunk-segments and their appendages) in many segmented 

 or polymerous animals, i.e., those which have passed beyond the 

 trochozoon stage or type. 



