34 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



the classical object for the study of the development of Ophiurids, has total 

 cleavage. However, the cleavage is irregular and the entoderm is formed 

 by delamination. 25 We have thus here a deviation from the normal process 

 of cleavage and entoderm formation tending towards the type occurring in 

 Isometra vivipara, Amphiura vivipara, and Cucumaria glacialis. Another 

 somewhat intermediate type is found in Henricia sanguinolenta (0. F. Miiller), 

 in which some nuclei may be found lying free in a larger part of the yolk not 

 yet limited into cells (c/. plate i, figure 11, in Masterman's 2C memoir). No 

 case quite analogous to what is found in the above-named forms of Crinoids, 

 Ophiurids, and Holothurians is known as yet among the Asteroids. In 

 Echinoids it would appear that the superficial cleavage is of quite general 

 occurrence among the forms which protect their young. I have been able 

 to ascertain it for Hypsiechinus coronatus Mortensen, Abatus cavernosus 

 (Philippi), and Amphipneustes kcehleri Mortensen. 



In Caswell Grave's paper on Ophiura brevispina, quoted above, it is 

 stated (p. 428) that the eggs of "practically all species of Echinoderms" are 

 minute bodies, containing only a small amount of yolk, which "seems to 

 interfere but slightly if at all with either the activities of the cells of the 

 developing larvse or with other processes of development and differentiation." 

 Although it will, evidently, be in exceptional cases only that the yolk sub- 

 stance causes a meroblastic development, large and yolk-laden eggs are by 

 no means exceptional in Echinoderms. Such eggs appear to be the rule in 

 all the Dendrochirote Holothurians; then very many Asteroids have large, 

 yolk-laden eggs, and it is evidently also the rule among the deep-sea Echino- 

 derms of all groups. So far as my experience goes, it would appear that 

 among Echinoderms as a whole the forms with large eggs rich in yolk are 

 about as numerous as those with minute, clear eggs, containing only a small 

 amount of yolk. The same must hold good for the typical pelagic larvse, this 

 being in correlation with the size of the eggs. 



2. FORMATION OF ENTEROOEL AND HYDROCCEL. 



The differentiation of the archenteron proceeds in the same way as in 

 Antedon and Tropiometra. A median constriction separates the archenteron 

 in an upper or anterior part, the entero-hydrocoel, and a lower or posterior 

 part, the ccelomic vesicle. In plate xiv, figure 11, the two parts are seen 

 still in connection. A slightly younger stage is figured in plate xiv, figure 6, 

 representing a sagittal longitudinal section. In the stage represented in 

 plate xiv, figure 7, the separation has been completed, a little downward 

 prolongation from the anterior part indicating the former connection between 

 the two parts. The posterior part has already divided into two lateral 



26 A. Russo, Embriologia dell' Amphiura squamala Sars, Att.d. R. Accad. d. Sci. fis. e matem. 

 Napoli. Ser. 2 a, vol. v, 1891. 



2S A. T. Masterman, The early development of Cribrella oculata (Forbes), with remarks on Echinoderm 

 development. Trans. Roy. Soc. Edinburgh, XL, 1902, p. 377. 



