What is generally considered, but perhaps wrongly, to be the most primitive mode of segmentation is seen in an oosperm, which is alecithal, i. e. devoid, or nearly so, of food-yolk. The nucleus divides with mitosis, and a constriction splits the oosperm into two equal or sub-equal halves or blastomeres. Each half then divides again into two, and so on. The two first divisions take place in a vertical plane, the third in a horizontal, the fourth in a vertical, and the fifth in a horizontal, and then regularity is lost. In many instances, however, a regular sequence of stages is not recognisable. The result of segmentation is the formation of a hollow sphere, the blastula or blastosphere, the cells or blastomeres being disposed in a single layer round a central cavity filled with an albuminous liquid, the blastocoele or segmentation cavity. In many instances the blasto-coele is absent, or nearly so, and the term morula is then used instead of blastosphere1. The cells of the blastosphere are frequently (? always) dissimilar at opposite poles, one set typically smaller and more clear, the other larger and more granular. The latter undergo invagination or embole, that is to say they sink inwards, obliterating more or less completely the blastocoele.
The result is an invaginate, or embolic Gastrula, an ovate or spherical body composed of a double layer of cells, an outer epiblast ( = ectoderm), an inner hypoblast ( = endoderm), separated or not by a space, the remnant of the blastocoele, and continuous at the blastopore or Gastrula mouth. The central cavity into which the blastopore leads is the archenteron 2.
Instances of typical or equal segmentation are met with in most groups of multicellular animals, but the process is commonly modified by the accumulation of food-yolk; the blastocoele may be absent or only slightly indicated, and transformation into a Gastrula is sometimes carried out by an invagination, sometimes by a modified form of invagination known as overgrowth or epibole, that is to say the epiblast grows round the hypoblast or yolk. The food-yolk may accumulate at one extremity of the oosperm, or in its centre; to these two types the terms telolecithal and centrolecithal are respectively applied. A telolecithal ovum may segment completely but unequally, the hypoblastic cells being larger and dividing more slowly, or its segmentation is partial and confined to a disc at one pole, a large amount of yolk remaining unsegmented; nuclei appear in it, however, at the pole of segmentation, a certain amount of protoplasm becomes segregated round them, and cells are thus added to the blastoderm or segmented area. The extremes are connected by transitional forms. Centrolecithal ova are confined to Arthropoda. The central aggregation of the yolk may be present from the first, or take place during segmentation.
The yolk is in the latter case always massed at the central ends of the blastomeres, which may or may not fuse, whilst in the former case the furrows between the blastomeres are superficial, i. e. do not penetrate to the centre of the oosperm. The central mass of yolk thus left either does not segment at all or does so at a late period, and the masses to which it gives origin are non-nucleate. The blastomeres may be equal or unequal, their formation may be simultaneous or successive, and is very often preceded by a multiplication of nuclei. Indeed in Peripatus cells are never distinctly delimited, the result being a syncytium1. The Gastrula is derived either by invagination or by differentiation of the yolk-cells.
1 The term 'morula' is also applied to solid masses of cells produced by segmentation and not yet definitively arranged in Gastrula-fashion.
2 The blastocoele is sometimes open by one or more pores to the exterior, e. g. in the amphi-blastula of Sycon (Sycandra). It would have been better if the term blastopore had been restricted by usage to such openings, and some such term as 'gastropore' applied to the Gastrula mouth.
In some Coelenterata the Gastrula stage is attained either by immigration of cells from one or various points of the blastosphere into the blastocoele, with subsequent differentiation of the immigrant cells, by delamination of the inner ends of the blastospheral cells, or by a mixture of the two processes. See p. 746, note I, pp. 752, 764, 800. The variations observable in closely allied genera, make it probable that the phenomena as observed in these cases are of secondary origin, due perhaps to a precocious formation of the endoderm. The name 'parenchymula' or 'parenchymella' has been applied to the form where the central cavity is filled with cells.
There can be no doubt that segmentation and gastrulation, processes which take place in every life-history or ontogeny, represent ancestral stages in the evolution or phylogeny of multicellular animals. But at the present time there is no such thing known as a sphere or blastosphere leading an independent life and reproducing its kind, its component cells united by a bond indissoluble without entailing death on each cell. The same statement is true of the Morula and of the Gastrula. The claims of the Mesozoa to represent a Gastrula are excessively doubtful; those of Haeckel's Gastreadae, at present inadmissible. See pp. 817-8.