8 UEPOUT ON THE SPAWNINC OF THE COMMON SOLE. 



sions on sucli subjects as tlie morphology of the notochord and the 

 mesoblast, &c., into wliich some of those have drifted who have failed 

 to avail themselves of this morphological anchor, with all that follows 

 its acceptance ; and I have, consequently, been on the qui vive for any 

 evidence which might serve to prove the point definitely to its gainsayers, 

 although feeling, personally, that the study of the living Elasmobranch 

 blastoderm for days and weeks, and of the Teleostean egg for hours 

 (supplemented if necessary by sections) gives ocular demonstration of 

 concrescence. 



Now the sole egg, I believe, is capable of giving such proof. The 

 curious aggregates of small oil globules, characteristic of the sole egg, 

 are well known, but I am, I believe, the first who has had the good 

 fortune to study the early developmental history under favourable 

 conditions, and thus to recognize that if suitable eggs be selected, and 

 isolated, and carefully sketched with the camera at short intervals, 

 these oil aggregates, which are at first distributed mainly in a zone 

 below the equator, can {a) before the formation of the embryonic ring, 

 be used as fixed points, and serve to show, at least within a small angle, 

 the relation of the plane of symmetry of the embryo to the first cleavage 

 planes, and (h) after the first formation of the ring, when they become 

 involved in those relative movements of different parts of the egg, 

 which are usually spoken of as epibolic gastrulation and concrescence, 

 (by those who accept the " Concrescence Theory " ) may be used, so to 

 speak, as floats whereby to follow these movements. 



Having noticed this fact, I thought I could not make better use of 

 the splendid material by the kindness of the Director so freely placed 

 at my disposal, than by concentrating my attention chiefly on that 

 early period of developmental history (the first two days or first day 

 and a half, according to whether the eggs are hatched in seven days or five) 

 during which these and other problems of fundamental morphological 

 interest are to be studied. 



I hope shortly, when I have supplemented my serial camera-sketches 

 of different stages of the same living eggs, by sections of corresponding 

 stages from my preserved material, to publish something fuller on the 

 above two points, and on other matters, such as the structure of the 

 egg, the morphological relations of the disc, the parablast and the yolk, 

 and gastrulation. For the jiresent, I may state that : — 



/. — 2'hc plane of symmetry of the cmhryo docs not hear one and the 

 same fixed relation in all cgrjs to the first segmentation plane. Thus out 

 of eleven eggs I found that in three the plane of symmetry of the 

 embryo coincided with the first segmentation plane, in four with the 

 second segmentation plane, and in four with a plane bisecting the angle 

 between these. 



