MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 47 



XI. OPISTHENOGENESIS, OR THE DEVELOPMENT OF SEGMENTS, MEDIAN 

 TUBERCLES, AND MARKINGS A TERGO. 



Weismaiin. in his suogestive •' Studies in the theoiT of descent" (1S7B), was the first to dis- 

 cuss the orii^in of the inarkinos of caterpilkrs, and to show that in Deih'ph'da luppophaeH the 

 ring-like spots of tiie larva ''first orig-inated on the segment bearing the 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 bod}' 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 Wilrtenberger,* 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 number of the whorls." 



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

 Cnemidophoi'HS te«>iellatu>< and (jnlai'ix, the breaking up of the striped coloration into transverse 

 spots begins first at the sacral and lumbar regions: "' the confluence of the spots appears there 

 first." 



We may cite some examples of this law of growth a tetyo, or opisthenogenesis. as it might 

 be called, which have fallen luider our own observation. 



In Daxylophia anguina, as shown by the figui'es 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 fi'ont part of the sui'anal 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 sjDots arise on the suranal plate. 



It thus appears that in the ontogeny 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 Syiii)nerist(( alb {fro ns the same phenomenon occurs. In stage I, as stated in my monograph 

 (p. 180), on each side of the ninth segment, is a large black, couuna-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 spots are 

 added on the side of the body 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 this 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 

 hinder segments accelerated or specialized. It thus appears that the_more advanced or ontoge- 

 netically later stvle of ornamentation originates at the end of the body. 



A parallel process 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 process of fusion of the two dorsal tubercles of the first 

 larv^al 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 bod}'. 



« Untersuchungen iiber das Variiren der ilauereidechse. Archiv f. Naturg., 1881. Ueber die Zeichnung der 

 Thiere. Zool. Anzeiger , 1882, 1883, 1884. Organic Evolution. London, 1890. 



6 A new contribution to the zoological proof of the Darwinian theory. Ansland, 1873. Xos. 1, 2, and .Stmlies 

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



Vol. 9 -0.5 4 



