The ova of some Teleostei are intermediate between the two types.
The ovum of the wood-louse, Oniscus murarius[49], may be taken as an example of the second type of meroblastic ovum. In this egg development commences by the appearance of a small clear mass with numerous transparent vesicles. This mass is the protoplasm which has become separated from the yolk. It undergoes segmentation in a perfectly normal fashion. Examples of other cases of this kind have been described by Van Beneden and Bessels[50] in Anchorella, and in Hessia by Van Beneden[51]. It appears from their researches that the protoplasm collects itself together, first of all in the interior of the egg, and then travels to the surface. It arrives at the surface after having already divided into two or more segments, which then rapidly divide in the usual manner to form the blastoderm.
There are some grounds for thinking that the cases of partial segmentation in the Arthropoda are not really quite comparable with those in other groups, but more probably fall under the next type of segmentation to be described. The grounds for this view are mentioned in connection with the next type.
In most if not all meroblastic ova there appear during and after segmentation a number of nuclei in the yolk adjoining the blastoderm, around which cells become differentiated. ([Figs. 46] and [47].) These cells join the part of the blastoderm formed by the normal segmentation of the germinal disc. Such nuclei are formed in all craniate meroblastic ova[52]. In Cephalopods they have been found by Lankester, and in Oniscus by Bobretzky. They have been by some authors supposed to originate from the nuclei of the blastoderm, and by others spontaneously in the yolk.
Some of the earliest observations on these nuclei were made by Lankester[53] in the Cephalopods. He found that they appeared first in a ring-like series round the edge of the blastoderm, and subsequently all over the yolk in a layer a little below the surface. He observed their development in the living ovum and found that they “commenced as minute points, gradually increasing in size like other free-formed nuclei.” A cell area subsequently forms around them.
By E. van Beneden[54] they were observed in a Teleostean ovum to appear nearly simultaneously in considerable numbers in the granular matter beneath the blastoderm. Van Beneden concludes from the simultaneous appearance of these bodies that they develop autogenously. Kupffer at an earlier period arrived at a similar conclusion. My own observations on these nuclei in Elasmobranchii on the whole support the conclusions to be derived from Lankester’s, Kupffer’s and Van Beneden’s observations. As mentioned above, the nuclei in Elasmobranchii do not appear simultaneously, but increase in number as development proceeds; and it is possible that Van Beneden may be mistaken on this point. No evidence came before me of derivation from pre-existing nuclei in the blastoderm. My observations prove however that the nuclei increase by division. This is shewn by the fact that I have found them with the spindle modification ([fig. 46], nx´), and that in most cases they usually exhibit the form of a number of aggregated vesicles[55], which is a character of nuclei which have just undergone division. It should be mentioned however that I failed to find a spindle modification of the nuclei in the later stages. Against these observations must be set those of Bobretzky, according to which the nuclei in Oniscus are really the nuclei of cells which have migrated from the blastoderm. Bobretzky’s observations do not however appear to be very conclusive.
It must be admitted that the general evidence at our command appears to indicate that the nuclei of the yolk in meroblastic ova originate spontaneously. There is however a difficulty in accepting this conclusion in the fact that all the other nuclei of the embryo are descendants of the first segmentation nucleus; and for this reason it still appears to me possible that the nuclei of the yolk will be found to originate from the continued division of one primitive nucleus, itself derived from the segmentation nucleus.
The existence of these nuclei in the yolk and the formation of a distinct cell body around them is a strong piece of evidence in favour of the view above maintained, (which is not universally accepted,) that the part of the ovum of meroblastic ova which does not segment is of the same nature as that which does segment, and differs only in being relatively deficient in active protoplasm.
The following forms have meroblastic ova of the first type: the Cephalopoda, Pyrosoma, Elasmobranchii, Teleostei, Reptilia, Aves, Ornithodelphia (?). The second type of meroblastic segmentation occurs in many Crustacea, (parasitic Copepoda, Isopoda Mysis, etc.). It is also stated to be found in Scorpio.
The ova of the majority of groups in the animal kingdom segment according to one of the types which have just been described. These types are not sharply separated, but form an unbroken series, commencing with the ovum which segments uniformly, and ending with the meroblastic ovum.