218 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



first seen. The disk, therefore, which orginally had a flat and then a 

 convex upper side, now begins to be considerably hollowed oat in the 

 middle, so that, at last, it presents the shape of a thin helinet-like cov- 

 ering round the lower part of the yelk. Seen from below, the egg now 

 shows the disk consisting of two leaves, (the vegetative and the animal,) 

 an inner lighter zone, and a more compact circular rim, which soon 

 appears on that side, where the above-mentioned thickening took place, 

 broader and more compact than on the other sides. During the further 

 development, the disk (statoblast) rapidly increases in size, encircling a 

 larger and larger portion of the yelk ; the outer rim produces a triangu- 

 lar continuation turned inside, which with its lower pointed end, gradu- 

 ally approaches the lower part of the egg, so that the inner lighter zone 

 of the disk assumes more and more the shape of a crescent. In this 

 continuation the incipient embryo can very soon be seen quite distinctly, 

 even before the disk has surrounded half of the yelk. First, a faint 

 longitudinal elevation is observed, thicker at the lower end, on the 

 sides of which two hemispherical projections can be seen indistinctly. 

 This longitudinal elevation is the spinal marrow of the embryo; the 

 lower and more compact portion is the head, or, properly cpeaking, the 

 brain ; and the two lateral projections are the beginning of the eyes. 

 During the eighth day after the impregnation, the disk may be seen 

 surrounding the whole of the yelk with the exception of a small portion 

 of the upper part, which appears like a ring-shaped opening surrounded 

 by a thicker edge. At the same time the triangular continuation has 

 become considerably elongated and has assumed the form of a narrow 

 ribbon, which stretches almost from one end of the egg to the other. 

 On the inside of this ribbon, but in the upper portion of it, the embryo 

 is now seen quite distinctly, the extremity of the tail being in immediate 

 connection with the disk, or rather with the ring encircling it. 



The third period in the development of the egg may properly be placed 

 as the time when the disk or skin has completely enveloped the yelk. 

 This phenomenon is accompanied by other essential phases of fhe devel- 

 opment, as several organs of the embryo, which before this could not be 

 seen, now first begin to show themselves, such as the lens of the eye, 

 the chorda dorsalis, the ear-bladders, the liver, the breast-fins, and the 

 heart. The beginning of the heart is seen by a faint swelling in the 

 region of the neck back of the eyes, in which a small circular bladder 

 is perceived, which, however, as yet shows no sign of any movement. 

 This bladder soon changes into a hollow cone placed obliquely on the 

 embryo, and shows a few irregular contractions, till at last it commences 

 its peculiar rhythmical movements. At the same time may be noticed 

 the first movements of the embryo itself inside the egg. These, at first, 

 consist of a faint, almost imperceptible trembling, which at greater or 

 less intervals is repeated in a more energetic manner. The pigment 

 now begins to show itself distinctly on the iris of the eyes in the shape 

 of small dots, and on the rest of the body as irregular stripes. The 



