III. Asteroidea. 15 



protoplasm (spermosphere) . The terminal nuclei develope fastest forming conical 

 masses of protoplasm which become gradually elongated (spermatoblasts, , with 

 peripheral nucleoli. The latter are eventually detached when the tailed 

 spermatozoids are liberated from the cellular rachis (except in Cribrella] . A 

 few of the proximal nuclei are not immediately transformed into spermatoblasts, 

 but remain to give rise to new spermospheres by their repeated subdivision. The 

 primordial cells of the ovary are contained in a network of interlacing connective 

 tissue fibres and are identical with the lymphatic cells of the ovoid gland. Some 

 remain unaltered, while others increase largely in size, each one pushing before it the 

 connective tissue network and primordial cells which are applied closely to it and 

 produce the appearance of a follicle epithelium. In the Asterinidae and Echinaste- 

 ridae coloured refractile vesicles appear in this connective tissue and form a sup- 

 plemental vitellus, which somehow enters the ovum, possibly for the purpose of 

 direct development. In the immature Asterina gibbosa the genital caeca contain 

 both spermospheres and imperfectly developed ova ; but the former are not replaced 

 after one discharge of spermatozoa, and the yellow supplemental vitellus appears 

 in the ova. One hermaphrodite Asterias glacialis was also met with. - - Asexual 

 reproduction may occur by median or radial division. The former process 

 occurs in Asterina ivega, Asterias tenuispinus and calamaria, the Starfish dividing 

 into 2 nearly equal halves ; while radial division by separation of single arms is 

 most marked in the Linckiadae, producing comet- forms. 



Fewkes describes the development of the calcareous plates in the young 

 Asterias. The first plates to appear on the abaclinal hemisome of the Brachiolaria 

 are the terminals and genitals (or basals) . They lie in 2 U-shaped lines one within 

 the other, the terminals, in the larger IT, being probably formed first. Between 

 the fifth terminal and the first genital, which ultimately lodges the madreporite, is 

 a relatively large space, the brachiolarian notch. This gradually closes as the 

 Brachiolaria is resorbed and the 2 Us become rings around the dorsocentral which 

 appears at first near the fifth genital. It is farthest from the blastopore in the 

 youngest larvae, and soon after its formation paired spines appear on the ter- 

 minals, being the earliest spines formed in the Starfish. The terminals also bear 

 the first pedicellariae which appear when 4 median dorsals are formed. The cal- 

 cifications of both pedicellariae and spines are independent of those of the plates 

 bearing them, and those of the pedicellariae are longitudinally double. Three 

 spines appear on the genitals before the Brachiolaria is resorbed, and the dorso- 

 central long bears a single prominent spine which is relatively larger than in later 

 stages, but others appear subsequently. The first dorsal (radial) does not appeal- 

 till 2 pairs of ambulacrals are formed ; other spiniferous dorsal plates are added 

 in the radial line outside it, and also smaller plates inside the circle of genitals. 

 The first marginals, each with a single spine, appear before the second median 

 dorsals, and are closely followed by the adarnbulacrals, also with single spines, 

 which at first alternate with the ambulacrals. The oral ambulacrals, representing 

 the spoon-shaped plates of Amphiura, are the first plates formed after the 1 1 on 

 the abactinal surface, appearing as 5 pairs of elongated spicules before resorption 

 of the brachiolar arms. The interbrachial ends of these plates in adjacent arms 

 grow towards each other forming two parallel ends in each interradius, of which 

 each bears two spines. These, though not separate calcifications, represent the 

 oral adambulacrals ; they appear after the first and before the second ambulacrals 

 of the rays. All these plates are developed independently and not as downward 

 growths from the abactiual surface. The odoutophores which represent the orals 

 of Amphiura, appear before the second ambulacrals, and are visible for a time on 

 the actinal surface as is permanently the case in deep-sea Asterids. Calcifications 



f* 



