JTW's Evolutionary Origins - Authors: Tauber, Alfred I.; Chernyak, Leon
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Elie Metchnikoff's Parenchymella/Phagocytella Theory

"Metchnikoff, however, discovered among primitive multicellular animals, such as sponges, hydroids, and lower medusae, a stage of development still more simple than the gastrula: this stage is without a digestive cavity and only assumes the gastrula form in its ulterior evolution. He also made the remarkable discovery that, in the most primitive multicellular animals, the endoderm is formed, not by means of invagination, but by the migration of a number of flagellated cells from one pole of the wall of the blastula into the central cavity. These cells draw in their flagellum, become amoeboid and mobile, multiply by division, fill the cavity of the blastula, and become capable of digesting. They originate the digestive cells of the complete organism and give birth to the mesoderm, which explains how the latter comes to contain the devouring cells even though these do not constitute the digestive organs properly so-called. Metchnikoff gave to that stage the name parenchymella, for the migrating cells constitute the endoderm in the condition of the parenchyma. The invariable prescence of this stage in the simplest multicellular animals, them primitive amoeboid state of the endodermic cells, cases of ulterior transformation of the parenchymella into the gastrula form in certain animals, the abscence of a differentiated digestive cavity, all proved that, according to Metchnikoff, that the parenchymella is more primitive than the gastrula, and is therefore entitled to be considered the prototype of multicellular beings. He saw confirmation of this in the fact that primitive adult animals also have no digestive cavity but merely an intracellular digestion (sponges, turbellaria). He concluded that the common ancestor of multicellular beings was a being constituted by an agglomeration of cells without a digestive cavity but endowed with intracellular digestion, like that of the 'parenchymula' stage of development. He therefore gave to that hypothetical ancestor the name parenchymella. Later, in 1886, he definitely formulated his theory of the genesis of multicellular beings, and having already stated the phagocyte theory, he substituted for the name parenchymella that of phagocytella, which indicated at the same time the primitive mode of digestion of that hypothetical ancestor. Reduced to its simplest form, it is presented, according to Metchnikoff, a certain analogy with a colony composed of unicellular beings of two kinds: the first, flagellated, forming the external layer, and the others amoeboid, occupying the centre of the colony and capable of digestion."
(Olga Metchnikoff, 1921; in Tauber & Chernyak, 1991)

"At the beginning, an inequality, that was to lead to a further differentiation, had to arise between individuals of the colony [referring to a Flagellata colony as the origin of the hypothetical primary multicellular organism]: while some individuals were meainly engulfing food, others were more adapted to locomotion and attraction to food. For mobile colonies, there was a certain advantage for individuals containing food particles (i.e., loaded and therefore heavier) not to remain on the periphery but to move closer to the centre. Another advantage was that feeding individuals were in conditions permitting [more perfect] accomplishment of their function. It is known that many Flagellata turn (from the state of monad) into the amoeboid state when they are feeding,... Further, it is easy to imagne that feeding individuals reproduced more frequently; so, a certain correlation between a more intensive feeding and reproduction could be formed. One more reason for the immigration was the immpossibility for the colony to broaden its surface beyond a certain limit, a fact of particular significance among swimming colonies. Because an increase in the individual's number made it possible to increase the [general] activity [of the colony], it was doubtlessly favorable when the cells, which could not find their place on the surfce, adapted to an existence within the colony. It is likely that for a long period, the individuals of the same colony differed from each other only quantitatively: the locomotive cells obtained food particles by movement of the flagella and themselves engulfed the smallest of the particles. Even in our time, it is possible to observe in some Coelenterata random engulfment of food by ectodermal cells. On the contrary, the inner amoeboid individuals were able to engulf larger particles which could not be caught by the locomotive elements. It is likely that during the process, the amoeboid individuals almost reached the periphery and (through multiple pores of the external layer) caught particles at the surface of the colony... Gradually, differentiation in the described direction reached increasingly higher levels; the locomotive cells were more or less losing their function of capturing food; the function increasingly adopted by the amoeboid phagocytes; the finest random pores between locomotive elements could be enlarged and turned into pores that in great number are found on the surface of sponges. To the extent of the proceeding individualization of the colony (the individual of the second order), the surface individuals differentiated into the ectoderm (the kinoblast), meanwhile, the amoeboid inner individuals differentiated into the phagocytoblast (the parenchyma or the mesoentoderm). The cells of the latter, in the case of their inability to engulf larger food particles, formed around them the plasmodium, similar to that as occurs now in the entoderm of siphonophores and in the mesoderm of many animals. The increase in activity of the multicellular organism (provided by the two primary organs) led to the satisfaction of the requirement of food; it is likely that larger plant and animal organisms were eaten. In order to make feeding possible, one or a few entrance pores, which was to lead to a mouth formation, had to arise... I have earlier called the transitional form between the Flagellata and the Metazoa, form which the latter were descended, the parenchymella. Now I would like to rename it phagocytella (Phagocytella), because this name indicates a very characteristic peculiarity of the form... Phagocytella possessed two primary organs - the kinoblast and the phagocytoblast which were not yet seperated from each other in that distinct way in which the embryonic layers of the majority of Metazoa are seperated; it is likely that the replenishment of the phagocytoblast by the intruding cells of the kinoblast continued for a long time. In regards to developmental history, it is possible to say that the ova of phagocytella (it had to have sexual reproduction) underwent uneven cleavage, the blastomeres divided in three spatial dimensions, and the blastocoel (which was then gradually filled by individual cells and by central products of the cell division) rose early"
(Metchnikoff; in Tauber and Chernyak, 1991)

to be cont... P64
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