Some Gregarines are large, jointed cells, divided into two pieces, a protomerite and a deutomerite; they are clad with a cuticle, under which lies a layer of muscular fibrils. After conjugation they encyst, the nucleus divides, and they break up into numerous peculiarly-shaped boat-like structures, (pseudonavicellæ), which afterwards are set free as small, sickle-shaped embryos. These exceedingly small germ-cells afterwards develop into the very different, adult gregarine-cells.
If the characters of a species be associated with a hereditary mass, an actual substance that is handed on from the parent-cell to the offspring, it is clear that the infusoria-like vagrant young of the Acinetan, and the sickle-shaped embryos of the Gregarine possess it, although for some time they are quite unlike the parent organism. For at last they become an Acinetan or a Gregarine, exactly like the parent-cell from which they arose as embryos.
These circumstances, among unicellular organisms, are a weighty indication of the error of concluding, with Weismann, in the case of multicellular forms, that because cells are unlike in outward appearance, the hereditary mass, or, as I call it, the nuclear matter, within them is also unlike. Such an assumption would involve us in the greatest contradictions. For the supposition that the nucleus is the hereditary mass transmitting the characters of the species necessitates the conclusion, in the case of unicellular forms, that the hereditary mass remains in possession of all the rudiments of the cell while it passes through the various phases of its cycle of development. Otherwise, these phases would have to be acquired anew in each case. We must, therefore, represent the possibilities of exchange between the nucleus, in its capacity of bearer of the hereditary mass, and the protoplasm as being such that all the rudiments are not simultaneously in activity, but that some of them can remain latent for a time.
SECOND GROUP OF FACTS.—THE LOWER MULTI-CELLULAR ORGANISMS.
Although in the development of unicellular organisms the way by which like begets like is plain and intelligible enough, at least in the cases dealt with, it is different with multicellular organisms, which have reached a higher grade of development. Among them we have to do with a continuous process of development, in which the highly-differentiated, multicellular organism arises from an egg, and in turn gives rise to an egg, and so on in unending sequence. But the succeeding stages of the sequence are so exceedingly dissimilar in appearance that the question how one step of the series turns into the next, and, above all, the question how the similarity of organisms, separated by the egg-stage, can be transmitted through the egg-stage, form the deepest riddle offered to biological investigation. Here, in a completeness so wonderful that our intelligence can hardly apprehend it, are presented to us the qualities of the organic material of which cells are made. Here lies that dark secret into which the various theories of generation try to direct a beam of light, and seek to find out the direction in which explanation may be found.
An intermediate stage which may serve towards the explanation of these circumstances is presented by the lower multicellular organisms, such as threadlike algæ, fungi, and other simple creatures. In them cells arise by division from the egg or from the spore, and become united into an individual of a higher rank; these cells resemble one another so completely in appearance and in qualities that there can be as little doubt as in the case of unicellular organisms that they arose by doubling division.
It is certain, then, that there exist multicellular bodies, often consisting of many thousand cells, in which each part retains the qualities of the egg from which it arose by doubling division, and which, as that method implies, possess the rudiments of the whole of which each is a part.
In this category there naturally fall the multinucleated masses of protoplasm, sometimes highly organised, in which every nucleus, surrounded by a shell of protoplasm, is capable of reproducing the whole. I am thinking of the slime-fungi (Myxomycetes), with their peculiar formation of reproductive bodies; of the 'acellular plants,' which in some cases closely resemble multicellular species in their formation of leaf and root, and in their mode of growth, as, for instance, Caulerpa, the multinucleated Foraminifera and Radiolarians. For, according to our definition of the cell, a multinucleated organism potentially is a multicellular organism.
In this matter Weismann has assumed a position which leads to peculiar consequences. In his opinion, somatic cells and germ-cells were sharply distinct at their first appearance in evolution, and have remained so ever since. Transitional forms between them are nowhere to be found. It would be inconsistent with his theory of the germplasm had somatic cells contained germplasm as their idioplasm, even when the soma first came into existence. The phyletic origin of the somatic cells depended directly upon an unequal separation of the determinants contained in the germplasm. It would totally contradict his presentation if the somatic cells, even at their first origin in phylogeny, contained, in addition to their patent special qualities, the qualities common to the whole species in a latent condition.
Weismann's conception, therefore, implies that many of the lower multicellular organisms, having no somatic-cells, have no body. Take the closely-allied creatures Pandorina morum and Volvox globator, which Weismann himself brings forward as instances for his view; the latter has a body, the former has no body, as all its cells are able to serve for reproduction!