TWIN GASTRUL.E AND BIPINNARLE. 579 



The twin bipinnarise of Luidia are not directly comparable with 

 the double Echinus-rudiments noted by MetschnikofE (11) and 

 described in detail by MacBride (10), since the latter appear late 

 in development, and their formation is a consequence of the abnormal 

 persistence and differentiation of one particular organ, viz. a right 

 hydrocoele. The same thing is true of the changes characteristic of 

 double hydrocoele in the developing Asterias larva (7, p. 275). As 

 regards structure, suggestive analogies can be drawn between our 

 bipinnariae and the abnormal medusae described by Allmann (1) and 

 Browne (2). In normal embryology perhaps the most interesting 

 parallel is to be found in the development of the Annelid Lumhricus 

 trapezoides Duges, in which a double gastrula, giving rise to two complete 

 earthworms, is produced by fission of the segmenting cell-mass (9). The 

 converse process, namely, fusion of two ova, has been shown to be 

 possible before (16 Ascaris) and after (16 Ascaris, 3 Sphcerechinus) 

 fertilisation, and also during the blastula stage (12 Sphcerecliinus). In 

 general, such fusion tends to produce double monstrosities resembling 

 our bipinnariae, but sometimes a perfectly single organism of larger than 

 normal size is the result (16 ; 3). 



Causation. E. Haeckel (8, 1869) discovered that the segmented egg of 

 a Siphonophore (Crystallodes), if artificially divided, could give rise to 

 several partial embryos, and E. B. Wilson (15, 1893) found that during 

 the early stages of segmentation in Amphioxus each of the component 

 cells, if separated from the rest, could develop into a perfect gastrula, 

 while imperfectly double gastrulae occurred abundantly in cultures 

 which had been subjected to shaking during the tw^o-celled stage. A 

 series of such gastrulae is shown in 15, PI. XXXIV, Figs. 66-73, while in 

 PI. XXXVII, some of the partially double stages which led up to them 

 (four-celled, eight-celled, blastulae) are also illustrated. As is well known, 

 similar or allied phenomena have been demonstrated to occur in the 

 development of many other ova, and there is now an extensive body of 

 literature dealing with experiments on the subject. Without going into 

 details for other groups, we may note that the ova of Asteroids w^ere 

 early found to react to experiment in much the same W' ay as those of 

 Amphioxus, and, what is more remarkable, it was ascertained by Driesch 

 (3) that as late as the blastula stage either half of a developing ovum 

 {Asterias glacialis, Astropecten) bisected transversely or longitudinally 

 could give rise to a bipinnaria. Thus in Asteroids it appears that single 

 cells in the earliest stages or cell masses at a later stage can, if isolated, 

 produce whole larvae. On the other hand, if the cells or cell masses in 



