C. E. Beecher — Origin and Significance of Spines. 11 



single sector of growth. For instance, in the radial plications of 

 a Spondylus or Lima, the earliest and primitive spines are found 

 near the beak, while those on the ventral border of an adult 

 specimen are the latest and most highly developed (figure 30). 

 These successive stages, therefore, are in the direction of 

 growth, and may be called longitudinal. (2) By the radial 

 divergence of the ribs or plications and the interpolation of 

 additional ones at various intervals, as many transverse com- 

 pound series of spines finally appear along the periphery as 

 there are primary radii. Hence, in a given case, there may be 

 two radii continuing to the beak, then by interpolation there 

 are successively 5, 11, 23, etc., radii, the highest number being 

 found at the periphery (figures 19, 20). Moreover, by taking 

 the distal spines on these 23 rows, there result the same stages 

 of spine development as shown in the longitudinal series along 

 any primitive plication (figure 20). A Pelecypod shell like 

 Spondyhcs is here used to illustrate this process, but the appli- 

 cation may also be made to the Brachiopods as well as to the 

 conical non-coiled Gastropods. In a coiled form like a Cephal- 

 opod or an ordinary Gastropod, the longitudinal lines would 

 follow the whorls spirally, and the transverse lines would cor- 

 respond to the lines or increments of growth of the shell. 

 Species in which the radii are all introduced at an early stage 

 of growth (many species of Cardium, Pecten, Lima) or in 

 which the radii multiply by regular dichotomy would show, of 

 course, only the longitudinal series, for at the margin of the 

 shell, the radii would be of the same size and age, and the 

 spines uniform. 



The foregoing example illustrates an important principle of 

 ontogeny; namely, that in organisms which repeat various parts 

 during their growth, these parts will develop or pass through a 

 series of stages corresponding to the initial and subsequent 

 stages of the parts repeated. In this way, structures appear- 

 ing late in the ontogeny of the individual will present primi- 

 tive infantile and adolescent characters. Further development, 

 if such takes place, will pass through a progressive series of 

 ontogenetic changes, and if the stages of growth are by serial 

 repetition and thus are retained in the part, it will be found 

 that such stages can be correlated with those appearing early in 

 the life or history of the individual. Therefore, in studies of 

 this kind, it is possible to take a structure appearing at matur- 

 ity, and from it deduce or predicate as to what were some of 

 the early characteristics of the whole individual. This prin- 

 ciple is termed localized stages of growth by Jackson 37 , and was 

 first noticed by him in some investigations on Echinoderms. 



