The protoplasmic layer of the ovum is represented in black.
The early stages are very closely similar in all the types so far observed. Segmentation commences by the outer layer of the ovum, which throughout behaves as the active layer, forming a protuberance at one pole, which may be called the formative pole. Close below this protuberance is placed the nucleus. In the median line of the protuberance a furrow appears ([fig. 82] A), which gradually deepens till it divides the ovum into two. The granular layer follows the furrow so that each of the fresh segments, like the original ovum is completely invested by a layer of granular protoplasm. Each segment contains a nucleus. A second similar division at right angles to the first gives rise to four segments ([fig. 82] B), and the segments so formed become again divided into eight ([fig. 82] C). In the division into eight, which takes place in a vertical plane, the segments formed are of unequal size, four of them being much smaller than the others. The eight segments are arranged in the form of a slightly curved disc round a vertical axis—the future long axis of the body;—and there is a cavity in this axis which, like the segmentation cavity of Sycandra raphanus, is open at both extremities. The disc with its concavity on the side of the formative pole has the shape sometimes of an ellipse ([fig. 82] C) and sometimes of a rectangle, in which the four small spheres occupy the poles of the longer axis. A bilateral symmetry is thus even at this stage clearly indicated.
In the next phase of segmentation the granular layer surrounding each segment again forms a protuberance at the formative pole, but, instead of each segment becoming divided into two equal parts, the protoplasmic protuberance alone is divided off from the main segment. In this way sixteen spheres become formed, of which eight are large and are formed mainly of the yolk material of the inner part of the ovum, and eight are small and entirely composed of the granular protoplasm. The eight small spheres form a ring on the formative surface of the large spheres ([fig. 82] D).
The small spheres now increase very rapidly ([fig. 82] E), partly by division and partly by the formation of fresh cells from the large spheres; and spread over the large spheres, forming in this way an epibolic gastrula. They constitute a layer of epiblast. ([fig. 83] A.) The large cells in the meantime remain relatively passive, though during the process they divide, in some cases more or less irregularly, while in Eucharis they divide into sixteen. The axial segmentation cavity would seem during the process to become obliterated.
There is an important discrepancy between the statements of Kowalevsky and Agassiz as to the course of the growth of the small cells. According to Agassiz the small cells grow most rapidly at the formative pole and cover this before they meet at the opposite pole. The reverse statement is made by Kowalevsky. It would seem that the above discrepancy is due to an interchange on the part of the one or the other of these authors of the two poles of the embryo, in that according to Agassiz the formation of the mouth takes place at the formative pole, and according to Kowalevsky at the pole opposite to this.
Without attempting to decide between the above views, we shall speak of the pole at which the mouth is formed as the oral pole.
Fig. 83. Four stages in the development of Idyia roseola. (After Agassiz.)
s.c. sense capsule; st. stomodæum.