MEMOlllS OF THE NATIONAL ACADEMY OF SCIENCES. 47 



XI. OPISTHENOGENESIS, OR THE DEVELOPMENT OF SEGMENTS, MEDIAN 

 TUBERCLES, AND MARKINGS A TERGO. 



Weismaiiiu in his suo-g-cstivc "Studios in the tlieorj- of descent" (1876), was the first to dis- 

 cuss tiie orijriti of the mariviups of cattM'pillars, and to show that in Deilrphila In'ppophaes the 

 ring'-like spots of tlie larva " tirst oriyiiiatod on tlic segment hearing tiie caudal horn, and were 

 then gradually transferred as secondary spots to the preceding segments" (vol. 1, p. 277). 



Afterwards (1881-1890) Eimer " showed that in the European wall lizard " a series of mark- 

 ings pass in succession over the body from behind forward, just as one wave follows another, 

 and the anterior ones vanish while new ones appear behind." He speaks of this mode of origin 

 of the markings as the "law of wave-like evolution or law of undulation." In confirmation of 

 this process or law he cites the conclusions of Wi'irtenberger,'' who had long before (1873) observed 

 that "in ammonites all structural changes show themselves first on the last (the outer) whorl of 

 the shell, such a change in the following generations being pushed farther and farther toward 

 the beginning of the spiral until it prevails in the greater numl)er of the whorls." 



Cope, in his "Primary factors of organic evolution" (1896), also shows that in the lizards 

 CnemidopJiorus tessd/ata.s and (/idar/.i, the breaking up of the striped coloration into transverse 

 spots begins first at the sacral and lumbar i-egions: " the confluence of the spots appears there 

 tirst." 



We may cite some examples of this law of growth a te/r/o, or opisthenogenesis, as it might 

 be called, which have fallen under our own observation. 



In Dasi/Iop/iia anguinu, as shown by the figures in PI. XXI of this monograph, Pt. I, it will 

 be observed that in stages III, IV, and the last stage, the dark longitudinal lines become on the 

 eighth-tenth abdominal segments broken up into separate isolated dark spots. In the larva 

 before the second molt there are no spots on the ninth and tenth segments. In stage III, how- 

 ever — i. e., after the second change of skin, as stated in my monograph (p. 175) — four black 

 spots now appear on the front part of the suranal plate. In the last stage the reddish spots 

 on the eighth abdominal segment, which are detached from the lateral lines of stages I and II, 

 now become specialized into the two black, comma-like spots, with a linear spot above and 

 beneath; and two, sometimes divided into four, black spots arise on the suranal plate. 



It thus appears that in the ontogenv of this species the process of breaking up or origin of 

 the spots from the longitudinal lines takes place on the last three segments of the body. 



In Symmerista alhifroiis the same phenomenon occurs. In stage I, as stated in my monograph 

 (p. ISO), on each side of the ninth segment, is a large black, comma-shaped spot, the point directed 

 forward and downward, while behind there is a median black dot. After the first molt there 

 arises behind the dorsal hump two instead of one median black spots, and two black sj^ots are 

 added on the side of the bodj- near the base of the anal legs, i. e., two each on the 9th and last 

 segments. 



After the second casting of the skin the marking of the last three abdominal segments become 

 specialized; what on the body in front are parallel black and red lines being in tliis region now 

 represented by separate spots. Thus, as regards the marking, the anterior part of the body 

 remains ornamented with the primitive parallel lines, while the process becomes on the three 

 hitider segments accelerated or specialized. It thus appears that the more advanced or ontoge- 

 netically later st3-le of ornamentation originates at the end of the bod^'. 



A parallel ijrocess takes place with the formation of the caudal horn or hump. Thus in 

 Symmerista, Dasylophia. and other horned Notodontida' and members of other groups, the eighth 

 abdominal segment is the theater of the pi'ocess of fusion of the two dorsal tubercles of the first 

 larval stage into a single tubercle or horn ; so that this segment appears to be the center of a 

 process of specialization which does not take place on any other segments of the bodj'. 



o Untersuchungen iiber das Variiren der Mauereidechse. Archiv f. Naturg., 1881. Ueber die Zeichnung der 

 Thiere. Zool. Anzeiger , 1882, 1883, 18S4. Organic Evolution. London, 1890. 



''A new contribution to tlie zoological proof of the Darwinian theory. Ansland, 1873. Nos. 1, 2, and .Studies 

 on the history of the Descent of the Ammonites. Leipzig, 1880. (In German. ) 



Vol. 9— 0.T i 



