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



which forms the external capsule, and subserves a protective function. When the 

 embryo is sufficiently mature the capsule is burst, — the rupture being due, no doubt, 

 to the vigorous motions of the young fish, which in the case of Pleuronectes fiesus 

 generally emerges from the capsule by pushing out its tail. 



The Micropyle. — The zona radiata is pierced by the micropyle (PL II. fig. 19, raic.), 

 an aperture probably universally present in Teleostean eggs, and in these it varies 

 very little in structure and appearance. Thus in the salmon, trout, pike, ruff, perch, 

 bullhead, gudgeon, minnow, chub, and various species of Gastrosteus, Eansom's descrip- 

 tion accords almost perfectly with the micropyle, as seen in the cod, haddock, ling, 

 whiting, bib, flounder, dab, plaice, gurnard, and others. At a certain point the capsule 

 is distinctly thickened, and an internal conical elevation is formed, which, as Balfour 

 says (No. 10, p. 51), corresponds with an external funnel-like depression, while a 

 cylindrical canal connects the bottom of the funnel with the apex of the inner papilla.* 

 The thickened appearance of the capsule in the micropylar region is not produced simply 

 by the protruding hillock, and due to the crateriform depression outside ; but as Andre 

 (No. 4, p. 201) ascertained, and as may be easily demonstrated in the delicate translucent 

 ova of the Gadidse or Pleuronectidse, the capsule is actually thicker at this point 

 (PI. X. fig. 9). List shows the same feature in Crenilabrus tinea} Viewed from 

 above, three parts may be distinguished — a large outer annulus and a smaller inner 

 ring, with a central pore which is the opening of a cylindrical tube. In the trout 

 these measure, according to Andre, *015 mm., '008 mm., and - 005 mm. in diameter 

 respectively. The first-named ring is the rim of the external crater; the inner ring 

 marks the narrower, deeper portion ; while the central aperture is the essential part, the 

 true microlpyar canal, which is not, however, perfectly cylindrical, but midway along its 

 course distinctly enlarges, and then narrows again. This sinuosity observable in the canal 

 proper, Andre thinks, is produced by the ends of the pore-canals or radial striae which jut 

 out slightly into the lumen of the micropyle (No. 4, p. 201). That the micropyle is really 

 a depression, and not simply a puncture, is shown by the fact that the striations of the zona 

 radiata present an inclination towards the micropyle, which is increased as the aperture is 

 approached, and still more so down the walls of the crater, their outer ends being directed 

 towards the cavity of the depression, and forming projections into it as just stated. 



This inclination of the striae is shown by Ransom and others ; but His does not 

 indicate it in his figures of the ovum of Salmo fario and S. salar (vide No. 67, Taf. i. 

 figs. 7, 8, 9, and 10) ; and the same may be said of List in his recent paper. Connected 

 with the depression and thickening of the capsule around the micropyle, is the striking 

 appearance external to the larger annulus seen in the marine and fresh- water species of 

 Gastrosteus, where bold radiating striae pass away from the margin of the external crater 

 (vide Nos. 1 53 and 67, Taf. i. fig. 15), a feature less marked in the chub, in which the margin 



* Ransom speaks in Gastrosteus of the micropyle as projecting actually into the protoplasmic disc, and of a subse- 

 quent shortening of its funnel after fertilisation (No. 127, p. 450). 

 t Op. cit., p. 597, fig. 2, a. 



