48 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



When it does take place, and there i.s a specialized single tubercle on the first abdominal 

 segment, as in Notodonta, Nerice, and more especially in Hyparpax and Schizura, the process of 

 fusion of two tubercles into a single specialized one, as on abdominal segments 1 and 8, proceeds 

 from behind forward, as it were in waves of translation of the specialized growth force from 

 behind forward. 



This may clearly be seen in the tigures on PI. XXIV, showing the development of the single 

 hump in Hyjxirpar aurora. In tig. 1 the dorsal tubercles / in stage I are all separated; in tig. 2 

 those on the eighth abdominal segment have ail begun to unite at their bases before they have on 

 the lirst abdominal segment; they seem to be a little behind at first, though later on the hump 

 on the first segment becomes higher and larger than the caudal horn. 



If there were any doubt as to the relative period when the tubercles become fused in 

 Hyparpax, in Schhiwa leptlnoldt-s (PI. XXVI), it is very clearly shown by fig. 1 that the fusion 

 of the two tubercles forming the caudal hump, as we will call it, i. e., that on the eighth abdominal 

 segment, has taken place before any signs of such fusion have appeared in the pair on any of the 

 segments in front. 



When the ontogeny oi JVerice hidentata is worked out, it will l)e a matter of umch 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 Schisura unicornis, in which 

 there is the very unusual occurrence of a pair of short, thick spines on the vertex of the head 

 (Pt. I, PI. 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 last two 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 Ceratocampida^ and Saturniiche, 

 the process of fusion of the two dorsal tubercles alwaj's first begins on the eighth abdominal 

 segment. 



( )plsthenogenesis as regards the markings appears to be of a piece, or somehow connected, 

 with the opisthenogenetic origin in postembryonic 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 Polygordius, as stated bv Balfour ("A treatise on comparative em))ry- 

 ology," 1880, 1, pp. 271, 272): The conversion of the larva into the adult takes place "by the inter- 

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

 a small aims-l)earing 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 postembryonic acquisition of new characters (certain trunk segments and their appen- 

 dages) in many segmented or polymerous animals, i. e., those which have passed beyond the 

 trochozoon stage or type. 



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 Pol}'- 

 gordius, 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 

 continually 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. 



« Some problems of annelid morphology. Biological lectures delivered at the marine biological laboratory at 

 Woods Hole. 1891. p. 61. See also A. D. Mead. The early development of marine annelids. Journal of mor- 

 phology, XIII. May, 1897. p. 227-326. 



