486 SUMMARY OF CURRENT RESEARCHES RELATING TO 



homoeosis, follow the same course of development as the left middle 

 commissure. The great majority of double hydroceles may be 

 accounted for in this way, but the possibility must not be excluded that, 

 given initial instability, the influence of atavism and primary homoeosis 

 may sometimes be sufficiently strong to induce the formation of double 

 hydroccele apart from the action of environmental factors. 



The data from double hydroccele accord with the view that there has 

 been a fixed stage in the evolution of Echinoderms ; that fixation took 

 place in the middle line of the preoral lobe ; and that, as in Antedon, 

 freedom was obtained by loss of the attaching stalk. 



Twin Gastrulse and Bipinnariae of Luidia sarsi.* — J. F. Gemmill 

 describes various types of twin Luidia larva?, which may be classified 

 according to the same system as Double Monstrosities among Vertebrates, 

 the alimentary canal of the larvae being taken as their representative 

 axial structure. The causation depends on early partial separation of 

 cells or of cell masses, accompanied by a minimal interference with the 

 vitality of the whole. Doubling (partial or complete) of the gastrula 

 invagination is the great step on which the differentiation of twin bi- 

 pinnarige depends. This differentiation shows very markedly the work- 

 ing of " regulation " processes, in the course of which, when union of 

 structures -occurs, the union is always between structures of homologous 

 origin. Thus preoral and postoral bands, enterocoeles, and particular 

 regions of the alimentary canal unite each with its counterpart. 



Axial Gradients in Early Development of Starfish.! — C. M. Child 

 has found evidence of an axial gradient of susceptibility (to cyanide) in 

 the cytoplasm and nucleus of the egg of Aster ias forbesii. The existence 

 of a quantitative metabolic gradient is confirmed by observations on 

 the rate of the oxidative formation of indophenol in the egg. The 

 gradient coincides in direction with the axis determined by the excentric 

 position of the nucleus in the ovarian egg. The region of highest 

 metabolic rate in this axial gradient becomes the animal pole of the egg 

 and the apical region of the larva. 



The gradient probably persists through the cleavage stages, and is 

 very distinct in the blastula and gastrula. It finally disappears in the 

 bipinnaria larva as metamorphosis approaches. Somewhat less complete 

 evidence points to the existence of symmetry gradients, in which the 

 region of highest rate becomes the oral side of the larval body, the region 

 of lowest rate the aboral side. These metabolic gradients are directly 

 i« lated only to the larval axes. Towards metamorphosis they disappear, 

 and later the new axial gradients of the starfish body arise. 



Experimental Study of Cleavage in Sea-urchin Ova.J— Theophilus 

 S. Painter has experimented with the ova of Strongylocmtrotus, which 

 he subjected to violent shaking for a few (4-7) minutes after fertiliza- 

 i ion, with the result that in a varying proportion of cases the centrosome 



* Journ. Marine Biol. Assoc, x. (1915) pp. 577-88 (3 pis.), 

 t Amer. Journ. Physiol., xxxvii. (1915) pp. 203-19 (10 figs.). 

 X Journ. Exper. Zool., xviii. (1915) pp. 299-317 (3 pis.). 



