The type of unequal segmentation is on the whole the most widely distributed in the animal kingdom. There is hardly a group without examples of it.
It occurs in Porifera, Hydrozoa, Actinozoa and Ctenophora. Amongst the Ctenophora this segmentation is of the most typical kind. Four equal segments are first formed in the two first periods. In the third period a circumferential furrow separates four smaller from four larger segments.
This type is also widely distributed amongst the unsegmented (Gephyrea, Turbellaria), as well as the segmented Vermes, and is typical for the Rotifera. It appears to be very rare in Echinoderms (Echinaster Sarsii). It is not uncommon in early stages of the segmentation of the lower Crustacea.
For Mollusca (except Cephalopoda) it is typical. Amongst the Ascidia it occurs in several forms (Salpa, Molgula) and amongst the Craniata it is typical in the Cyclostomata, Amphibia, and some Ganoids, e.g. Accipenser.
Partial segmentation. The next type of segmentation we have to deal with has long been recognized as partial segmentation. It is a type in which only part of the ovum, called the germinal disc, undergoes segmentation, the remainder usually forming an appendage of the embryo known as the yolk-sack. Ova belonging to the two groups already dealt with are frequently classed together as holoblastic ova, in opposition to ova of the present group in which the segmentation is only partial, and which are therefore called meroblastic. For embryological purposes this is in many ways a very convenient classification, but ova belonging to the present group are in reality separated by no sharp line from those belonging to the group just described.
Fig. 44. Surface views of the early stages of the segmentation in a fowl’s egg.
(After Coste.)
a. edge of germinal disc. b. vertical furrow. c. small central segment. d. larger peripheral segment.
The origin and nature of meroblastic ova will best be understood by taking an ovum with an unequal segmentation, such as that of the frog, and considering what would take place in accordance with the laws already laid down, supposing the amount of food-yolk at the vitelline pole to be enormously increased. What would happen may be conveniently illustrated by [fig. 44], representing the segmentation of a fowl’s egg. There would first obviously appear a vertical furrow at the formative or protoplasmic pole. ([Fig. 44] A, b.) This would gradually advance round the ovum and commence to divide it into two halves. Before the furrow had however proceeded very far it would come to the vitelline part of the ovum; here, according to the law previously enunciated, it would travel very slowly, and if the amount of the food-yolk was practically infinite as compared with the protoplasm, it would absolutely cease to advance. A second vertical furrow would soon be formed, crossing the first at right angles, and like it not advancing beyond the edge of the germinal disc. ([Fig. 44] B.)