sg. segmentation cavity. A. Stage with two equal segments. B. Stage with four equal segments. C. Stage after the four segments have become divided by an equatorial furrow into eight equal segments. D. Stage in which a single layer of cells encloses a central segmentation cavity. E. Somewhat older stage in optical section.
In other instances, which however are rarer than those in which a segmentation cavity is present, there is no trace of a central cavity, and the sphere at the close of segmentation is quite solid. In such instances the solid sphere is known as a morula. It is found in some Sponges, many Cœlenterata, some Nemertines, etc., and in Mammals; in which group the segmentation is not however quite regular. All intermediate conditions between a large segmentation cavity, and a very minute central cavity which may be surrounded by more than a single row of cells have been described.
The segmentation cavity has occasionally, as in Sycandra, the Ctenophora and Amphioxus, the form of an axial perforation of the ovum open at both extremities.
When the process of regular segmentation is examined somewhat more in detail it is found to follow as a rule a rather definite rhythm. The ovum is first divided in a plane which may be called vertical, into two equal parts ([fig. 39], A). This division is followed by a second, also in a vertical plane, but at right angles to the first plane, and by it each of the previous segments is halved ([fig. 39], B.) In the third segmentation the plane of division is horizontal or equatorial and divides each of the four segments into two halves, making eight segments in all ([fig. 39], C). In the fourth period the segmentation takes place in two vertical planes each at an angle of 45° with one of the previous vertical planes. All the segments are thus again divided into two equal parts. In the fifth period there are two equatorial planes one on each side of the original equatorial plane, and thirty-two spheres are present at the close of this period. Sixty-four segments are formed at the sixth period, but beyond the fourth and fifth periods the original regularity is not usually preserved.
In many instances the type of segmentation just described cannot be distinctly recognized. All that can be noticed is that at each fresh segmentation every segment becomes divided into two equal parts. It is not absolutely certain that there is not always some slight inequality in the segments formed, by which, what are known as the animal and vegetative poles of the ovum, can very early be distinguished. A regular segmentation is found in species in most groups of the animal kingdom. It is very common in Sponges and Cœlenterates. Though less common so far as is known amongst the Vermes, it is yet found in many of the lower types, viz. Nematoidea, Gordiacea, Trematoda, Nemertea (apparently as a rule), Sagitta, Chætonotus, some Gephyrea (Phoronis); though not usual it occurs amongst Chætopoda, e.g. Serpula. It is the usual type of segmentation amongst the Echinodermata. Amongst the Crustacea it appears (for the earlier phases of segmentation at any rate) not infrequently amongst the lower forms, and even occurs amongst the Amphipoda (Phronima). It is however very rare amongst the Tracheata, Podura affording the one example of it known to me. It is almost as rare amongst Mollusca as amongst the Tracheata, but occurs in Chiton and is nearly approached in some Nudibranchiata. In Vertebrata it is most nearly approached in Amphioxus[36].
Most of the eggs which have a perfectly regular segmentation are of a very insignificant size and rarely contain much food-yolk: in the vast majority of eggs there is present however a considerable bulk of food material usually in the form of highly refracting yolk-spherules. These yolk-spherules lie embedded in the protoplasm of the ovum, but are in most instances not distributed uniformly, being less closely packed and smaller at one pole of the ovum than elsewhere. Where the yolk-spherules are fewest the active protoplasm is necessarily most concentrated, and we can lay down as a general law[37] that the velocity of segmentation in any part of the ovum is roughly speaking proportional to the concentration of the protoplasm there; and that the size of the segments is inversely proportional to the concentration of the protoplasm. Thus the segments produced from that part of an egg where the yolk-spherules are most bulky, and where therefore the protoplasm is least concentrated, are larger than the remaining segments, and their formation proceeds more slowly.
Though where much food-yolk is present it is generally distributed unequally, yet there are many cases in which it is not possible to notice this very distinctly. In most of these cases the segmentation is all the same unequal, and it is probable that they form apparent rather than real exceptions to the law laid down above. Although before segmentation the protoplasm may be uniformly distributed, yet in many instances, e.g. Mollusca, Vermes, etc., during or at the commencement of segmentation the protoplasm becomes aggregated at one pole, and one of the segments formed consists of clear protoplasm, all the food-yolk being contained in the other and larger segment.
Unequal Segmentation. The type of segmentation I now proceed to describe has been called by Haeckel (No. [105]) ‘unequal segmentation’, a term which may conveniently be adopted. I commence by describing it as it occurs in the well-known and typical instance of the Frog[38].
The ripe ovum of the common Frog and of most other tailless Amphibians presents the following structure. One half appears black and the other white. The former I shall call the upper pole, the latter the lower. The ovum is composed of protoplasm containing in suspension numerous yolk-spherules. The largest of these are situated at the lower pole, the smaller ones at the upper pole, and the smallest of all in the peripheral layer of the upper pole, in which also pigment is scattered and causes the black colour visible from the surface.