The bird's egg is particularly important for our purpose, because most of the chief studies of the development of the vertebrates are based on observations of the hen's egg during hatching. The mammal ovum is much more difficult to obtain and study, and for this practical and obvious reason very rarely thoroughly investigated. But we can get hens' eggs in any quantity at any time, and, by means of artificial incubation, follow the development of the embryo step by step. The bird's egg differs considerably from the tiny mammal ovum in size, a large quantity of food-yelk accumulating within the original yelk or the protoplasm of the ovum. This is the yellow ball which we commonly call the yolk of the egg. In order to understand the bird's egg aright--for it is very often quite wrongly explained--we must examine it in its original condition, and follow it from the very beginning of its development in the bird's ovary. We then see that the original ovum is a quite small, naked, and simple cell with a nucleus, not differing in either size or shape from the original ovum of the mammals and other animals (cf. Figure 1.13 E). As in the case of all the craniota (animals with a skull), the original or primitive ovum (protovum) is covered with a continuous layer of small cells. This membrane is the follicle, from which the ovum afterwards issues. Immediately underneath it the structureless yelk-membrane is secreted from the yelk. (FIGURE 1.55. Longitudinal section through the blastula of a shark (Pristiuris). (From Ruckert.) (Looked at from the left; to the right is the hinder end, H, to the left the fore end, V.) B segmentation-cavity, kz cells of the germinal membrane, dk yelk-nuclei. FIGURE 1.56. Longitudinal section of the blastula of a shark (Pristiurus) at the beginning of gastrulation. (From Ruckert.) (Seen from the left.) V fore end, H hind end, B segmentation-cavity, ud first trace of the primitive gut, dk yelk-nuclei, fd fine-grained yelk, gd coarse-grained yelk.)