Section 13. The phenomena of the extrusion of polar bodies and fertilization are treated of later, and will, therefore, not be considered now. We will start our description with an egg-cell, which has escaped, of course, since there are no genital ducts, by rupture of the parent, has been fertilized by the male element, and is about to develop into a young amphioxus. It is simply a single cell, with some power of amoeboid motion, a single nucleus and nucleolus; and in amphioxus its protoplasm is clear and transparent. Frequently ova are loaded with granules of food store (yolk), which enable the young animal to go far with its development before it is hatched and has to begin fending for itself. Such an ovum as that of our present type, however being devoid of such yolk (alecithal = without yolk), necessitates a very early start in life, and, for reasons too complicated to state fully here, the development in such a case is considered particularly instructive and primitive by zoologists. Section 14. The first thing to be seen in the developing cell is a deepening circular groove (Figure 1, Sheet 21), which divides the ovum into two parts. Another groove then cuts at right angles to this subdividing the two into four (Figure 2). Another groove, at right angles to both the former, follows, making the four eight (Figure 3). And so subdivision goes on. The whole process is called segmentation or cleavage. Section 15. At the end of segmentation we get a hollow sphere of small cells, the cells separating from one another centrally and enclosing a cavity as the process proceeds. This is the blastosphere, shown diagrammatically in Figure 4, and of which an internal view, rather truer to the facts of the case as regards shape, is given as Figure 5. The central cavity is the segmentation cavity (s.c.). Section 16. Invagination follows (Figure 6). In this process a portion of the blastosphere wall is the tucked into the rest, as indicated by the arrow, so that a two-layered sack is formed. The space ar. is the archenteron, the primordial intestine, and its mouth is called, the blastopore (bp.). The outer layer of this double-walled sac is called the epiblast. For the present we will give the inner lining no special term. The young amphioxus has, at this stage, which is called the gastrula stage, a curious parallelism with such a lowly form as the Hydra of our ditches. This latter creature, like the gastrula, consists essentially of two layers of cells, an outer protective and sensory layer, and an inner digestive one; it has a primordial intestine, or archenteron, and its mouth is sometimes regarded as being a blastopore. All animals that have little yolk, and start early in life for themselves, pass through a gastrula stage, substantially the same as this of amphioxus.