XVI ECHINODEEMATA 529 



size, differentiated into its typical regions, out of a fragment of a gut 

 of larger size, can be surmounted, as Zur Strassen showed, by the 

 assumption of " formative stimuli " proceeding from the cut ends of 

 the gut fragment, and determining by their interaction the formation 

 of constrictions in what was, at first, a uniform tube. 



The great difference, then, between the eggs of Echinoidea and 

 those of Nemertinea, Ctenophora, Annelida, and MoUusca is that in 

 the latter eggs the organ-forming substances are separated from one 

 another at a far earlier date in development ; that, in fact, in these 

 groups the segmentation of the egg is not merely the multiplication 

 of nuclei, as it is in Echinoidea, but is already incipient organogeny. 



HOLOTHUROIDEA 



Our knowledge of the development of Holothuroidea constitutes 

 the least satisfactory part of our knowledge of the embryology of 

 Echinodermata. A complete series of stages of the external form 

 is only known in two or three life-histories, viz. those of Synapta 

 digitata, Synapta vivipara, and Oucumaria planci. In only one of 

 these cases is a larva formed which leads a free-swimming ]ife for 

 a considerable time, and which can be compared to the Bipinnaria, 

 Ophiopluteus, and Echinopluteus larvae which have been described. 



The eggs of Oucumaria planci, and so far as we know of other 

 species of Oucumaria as well, are yolky, and undergo a shortened 

 development, in which the larva, although a free-swimming organism, 

 takes no food but depends for its nourishment on yolk grains stored 

 in its cells, like the larva of Solaster endeca. 



The young of Synapta vivipara undergo the whole of their 

 development within the body-cavity of the mother, and yet this is 

 the form which has been most carefully investigated and in which 

 modern methods have been most conscientiously applied. We owe 

 our knowledge of the development of this form to Lyman Clark 

 (1898), and his results seem to show that in most respects, so far 

 at least as the formation of internal organs goes, the development of 

 this form agrees with that of Synapta digitata. This latter we shall 

 select as type, but we must warn the reader that our knowledge of 

 its development is very scanty compared to our knowledge of the 

 development of Asterias ruhens, of Ophiothrix fragilis, or of Echinus 

 esculentus. 



SYNAPTA DIGITATA 



Our account of the early stages we owe to Selenka, who alone 

 has artificially fertilized the eggs of this form (1893). Semon has 

 given a highly defective account of its development and of its 

 metamorphosis, based on larvae caught in the Plankton, and has 

 made this account the ground for a most fantastic and improbable 

 theory of the phylogeny of Echinoderms (1898). Bury (1889 and 

 1896) devotes portions of two most valuable papers to the considera- 



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