732 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



J. T. Cunningham. Again, it is observed that towards the closure of the blastopore the 

 " limbs " of the blastoderm seem to go — so far — into the embryo. When this projection is 

 less marked the caudal end of the embryo may still destroy the regularity of the 

 circumference, as in PI. III. fig. 23, recalling the horse-shoe-shaped blastopore of Astacus, 

 such variations being easily explained by the bulk of the contained deutoplasmic matrix 

 and the tension of the blastodermic membrane. This pressure outwards, as Van Bambeke 

 pointed out, and the restraint of the blastoderm, frequently produce a contracted opening, 

 like the mouth of a balloon (see Van Bambeke's figure, No. 20a, pi. ii. fig. 9), from 

 which a plug of yolk protrudes, just as in the Crustacean ovum, mentioned above, an 

 endodermal protrusion fills up the blastopore. In Teleosteans, as in Astacus, the plug 

 diminishes as the blastopore closes. In the gurnard, as the blastopore closes, projecting 

 cells are seen, which often send out protoplasmic processes, those protruding from the 

 blastoporic lip somewhat resembling the processes which under pressure are pushed out 

 from the marginal cells of the blastodermic ring at an earlier stage (PI. II. fig. 16). The 

 time of the closure of the blastopore of course varies, according to circumstances, in 

 common with the other features of development. Thus in Trigla gurnardus the closure 

 was observed to be effected on the third day after fertilisation ; whereas in another series 

 earlier in the same season (May), the temperature being lower, this did not occur until 

 the fifth day. As closure takes place the yolk may often, in side views, be seen still to 

 project as a diminished yolk-plug (PI. III. fig. 15) ; but usually as closure is effected 

 the blastopore forms a trumpet-shaped opening, round which the deeply corrugated lip 

 rises as a circular eminence (PL III. figs. 9, 10, 21). 



Kupffer's Vesicle. — When the blastopore closes, or often a few hours earlier,* a minute 

 vesicle arises on the ventral aspect of the embryo slightly anterior to the caudal termina- 

 tion. Its advent is preceded by the appearance, in some cases, of vesicles or small 

 elongated spaces (PI. III. fig. 17), evidently filled with colourless or pinkish fluid. They 

 occur quite at the margin, as if the advancing embryonic area became elevated at these 

 points, and progressed over them. In other cases a granular thickening occurs in which 

 a few rounded vesicles are imbedded, as can be readily seen in Trigla gurnardus and 

 other forms shortly before the blastopore closes. Kingsley and Conn noted such a group 

 of minute vesicles, which in five hours apparently by coalescence showed the characteristic 

 form and appearance of Kupffer's vesicle. It is defined in their figure, above by hypo- 

 blast, and below by periblast (No. 78, pi. xvi. fig. 54). It is variable in the precise time 

 of its appearance, for Henneguy noticed it in Salmo fario when only about half of the 

 vitellus was covered by the blastoderm (No. 80). In Molva vulgaris, Gadus morrhua, 

 and other species it is usually not visible during the open state of the blastopore, but 

 both in position and time of its appearance it varies, though the clear vesicular structures, 

 with a delicate envelope, are usually exhibited. Kupffer, who first described it in 

 Gastrosteus, Gobius, and others, calls it the " allantois," and says that it acquires a coating 

 of cylindrical epithelium, and finally becomes the bladder, though he did not show how the 

 * J. T. Cunningham found that in Clupea it was late in appearing (January 1886). 



