of the Fishery Board for Scotland. 



251 



cell (PI. XII. fig. 2,pc). The processes leading to its protrusion have not 

 yet been made out. The earliest observed instances of the appearance of 

 the polar cell, after the mixing of the ova and spermatozoa, was about four 

 hours. It may, however, occur much sooner. The polar cell is typically 

 a highly refractive vescicle, projecting beyond the vitelline membrane, its 

 long axis in line with the radius. Sometimes a little clear cell may 

 project into it (PI. XII. fig. 4). It often contains refractive particles. 

 The distal portion may become constricted off, a partition being formed 

 between it and the proximal stalk-like portion. By the application of a 

 substance causing endosmotic action, the deutoplasm finds an easy exit into 

 the polar cell, which, if the action be continued, bursts and allows the egg- 

 contents to flow forth to the exterior (PI. XII. fig. 8, d). 



The following account of the segmentation is built up from many series, 

 not from continued observation of one ovum. Definite developmental 

 phases in the segmentation, such as are affirmed to occur in Modiolaria 

 marmorata, Gardium exiguum* and American oyster, f have not been 

 determined. In three hours or less, the ovum first segments into two 

 blastomeres or segments, — the macromere and the micromere. Normally, 

 the latter is about a third smaller than the former ; sometimes they are 

 equal in size. It is not mere bipartition, for, after the process, the macro- 

 mere is not much smaller than the unsegmented ovum. The segments are 

 nucleated. The egg capsule accommodates itself to the outline of the seg- 

 ments. The polar cell is situated in the sulcus between the segments (PI. 

 XII. fig. 10, pc)i In one instance there appeared lenticular refractive bodies, 

 apparently identical with those figured by Bobretzky J as found in Nassa 

 mutabilis (PL XII. fig. 11, la). Proliferation of the micromeral segments 

 goes on apace, following a course essentially similar to that pursued by 

 cardium and oyster, the result being that a body of very irregular outline, and 

 considerably larger than the original ovum, is produced (PI. XII. fig. 18). 

 In advancing to the blastosphere stage, the micromeral segments become 

 more numerous and smaller, and envelop the macromere. Meanwhile, 

 the opaque brownish deutoplasmic granules disappear, leaving the embryos 

 translucent. For a time the macromere is visible, enclosed by the cap of 

 blastomeres, its still exposed peripheral portion being now distinctly 

 crenate (PL XII. fig. 19, ma). At this stage the polar cell may still persist, 

 asymmetrically placed with respect to the macromere. Bipartition of the 

 macromere was noticed (PL XII. fig. 21, ma). Finally, becoming completely 

 enclosed by the micromeral segments, the macromere is coincidently broken 

 up into many comparatively large, distinct, hypoblastic cells. The blasto- 

 sphere is now spherical or subspherical. The approximation of the 

 micromeral cells gives rise to the blastopore (PL XII. fig. 22, bl) ; the 

 macromeral cells become arranged as the hypodermal layer • the now 

 translucent blastosphere acquires external cilia, and swims or rotates at 

 first near the bottom of the vessel. The cilia are very minute, and 

 apparently form a general coat. Soon a zone of much longer cilia appears 

 near one pole of the now somewhat elongated embryo, the pole being 

 occupied by one, or sometimes two, long, strong, flexible flagella (PL XII. 

 figs. 22 and 23,/). The trochosphere stage maybe attained in twenty- 

 eight hours. The eilium composing the velum vibrate rapidly tow 7 ards 

 and away from the pole, propelling the embryo rapidly forward. The 

 central flagellum is kept antenna-like in front. The fine cilia are still seen 

 to clothe a great part of the trochosphere. The fate of the blastopore was 

 not satisfactorily determined. It is apt to be confounded with the shell* 



* Loven, loc. tit. 

 t Brooks, loc. cit. 



X Vide Balfour (after Bobretzky), Compar. Embryology, vol. i. \ 



