Book - Russian Embryology (1750 - 1850) 4
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Blyakher L. History of embryology in Russia from the middle of the eighteenth to the middle of the nineteenth century (istoryia embriologii v Rossii s serediny XVIII do serediny XIX veka) (1955) Academy of Sciences USSR. Institute of the History of Science and Technology. Translation Smithsonian Institution (1982).
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Publishing House of the Academy of Science USSR Moscow 1955 Translated from Russian Translated and Edited by: Dr. Hosni Ibrahim Youssef # Faculty of Veterinary Medicine Cairo University Dr. Boulos Abdel Malek Head of Veterinary Research Division NAMRU-3, Cairo Arab Republic of Egypt Published for The Smithsonian Institution and the National Science Foundation, Washington, D.C, by The Al Ahram Center for Scientific Translations 1982
The Smithsonian Institution and the National Science Foundation, Washington, D.C by The Al Ahram Center for Scientific Translations (1982)
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Chapter 4 The "Theory Of Generation" of K. F. Wolff
Wolff's first work, with which he began to found the epigenetic theory and his struggle against preformation, was his Latin dissertation, THEORIA GENERATIONS, 1 to which his German book under the same title is related. 2 Wolff's factual data and his opinions appear strictly and systematically in THEORIA GENERATIONS . THEORIE VON DER GENERATION provides necessary additions and explanations.
Wolff's dissertation begins with the established plan of the whole work. First, Wolff determined "the development of the organized body" as the appearance of all parts of this body (§ 1). He sent a compliment to the "famous Haller," who, in Wolff's words, discovered the laws of development, i.e. found the influence of the power of the living body by which its formation is accomplished (§ 2) . The compliment to Haller cannot be explained by the real merits of the latter in the creation of the development theory. But its purpose can be seen from Wolff's letters to Haller, in which Wolff hoped to attract the famous physiologist to his side and to convince him of the correctness of his own opinions. This phrase about Haller stands in direct contradiction with the contents of the following thesis, in which Wolff spoke directly against the preformation (predelineation) theory, i.e. in fact against Haller. This famous thesis runs as follows (§ 3) : "The defenders of a predelineation system do not explain the phenomenon of generation, but confirm that this does not exist." In his dissertation, Wolff clearly, concisely, and completely expressed the correct idea that the preformation theory, assuming the preformation of the organism in the early embryo, as a matter of fact denies the real development formation of living creatures. Wolff made it his task to produce a theory of development, based upon principles and laws (5 5) , and which "can show sufficient basis of the origin of the living creature" (S 6) . The true theory of development must show the causes of development of the organism, i.e. must represent its philosophical cognition, that being the real science of organized bodies ($ 10) . According to Wolff, the doctrine about development, as related to anatomy, is purely a descriptive understanding of living bodies and the philosophical cognition of substances related to their historical cognition; therefore the theory of development can be called "rational anatomy" (§ 11) . In his German volume, Wolff detailed the contrast of historical and philosophical cognitions. The explanation of composition of a whole from parts and the description of these parts represents the task of the historical cognition. If the knowledge comes not only from direct experiment, but also from causes and bases, then this gives grounds for concluding that the substance must be this and not another, and that it necessarily possesses determined characters. If this is the case, then it will not be historical knowledge, but philosophical cognition.^ The theory of development must explain the phenomena of nutrition and growth, and also the formation of the constituent parts of the organism (§ 24) . It must take into consideration all essential and also less essential conditions, and it must investigate why this or that process of development proceeds in one way and not in another way ($ 25) .
Kaspar Friedrich Wolff, THEORIA GENERATIONS , Halae ad Salam, 1759, 146 pp. In the present work the Latin text is used and also its German translation, Leipzig, 1896, 95 + 98 pp.
Kaspar Friedrich Wolff, THEORIE VON DER GENERATION. IN ZWO ABHANDLUNGEN ERKLART UND BEWIESEN, Berlin: F. W. Birnstiel, 1764, 16 + 284 pp.
Already from Wolff's program, it is clear that he took upon himself a task of extraordinary importance: to give the general theory of the formation and development of living creatures and its contrast to the false theory of ontogenesis, which was unconditionally doctrinaire about preformation. He underlined the importance of a theory founded upon facts rather than upon groundless discussions. "I cannot agree in any case," he wrote, "that it can be performed by those.... who hold speeches about this subject, as if there were no science, truth and grace" (5 4, Introduction) .
3. THEORIE VON DER GENERATION, p. 8. 50
The first part of the dissertation is dedicated to plant development. In this part Wolff postulated in plants the presence of "power, which is fluid collected from earth, forced to act in the root, and distributed throughout the plant, part of which is consumed in different places, and part of which leaves the plant" ($ 1) . He called this power "the essential power of the plant" ($ 4) .
Before speaking about the effect of the essential power on development, Wolff considered the structure of the young rudiments of leaves, derived from buds. Using imperfect optical means, he could nonetheless see distinctly that this rudiment was derived from those structural formations which exist in the formed leaf, namely the fibers and vessels. The rudiment consists of transparent vesicles or, as in the seed of a bean, of a light, homogenous substance. At the beginning, vesicles are so few in number that they can be easily counted, and Wolff found not more than twenty in the young rudiments of the leaf. The number of vesicles increases ("soon they become innumerable" $ 8) , while the size of each vesicle remains as before, Very early, a few vessels appear in the apex of the stem, the number later increasing visibly.
The formation of new vesicles and new vessels takes place, according to Wolff, in the following way. The delicate homogenous substance filling the spaces between the vesicles is extended by the flowing nutritive juices. As a result of this, rounded cavities are formed, producing newly formed vesicles which are distributed between the early vesicles. This same movement of the nutritive juices builds canals in the homogenous mass which are transformed subsequently into vessels ($ 21) . The formative ability of the nutritive juice occurs during cessation of its movement, when it is transformed by evaporation into a thick, then viscous, and finally a solid substance. Wolff called this character of the nutritive juice the solidification ability ($ 27).
Wolff pointed out that there is no preexisting structure in the young rudiments of plant organs, because from the beginning their formed substance, consists of a simple mixture of substances and is deprived of any internal organization. Only after this, the vesicles and vessels are formed in this substance ($ 33) . The thickening and formation of the walls of the vesicles and vessels Wolff considered the result of the above mentioned movement of juices, which gradually lose their volatility. As a result, the solid substance is deposited in the walls of the extended rounded cavities (vesicles) and canals (the future. vessels) . Simultaneously, new vesicles and new rudiments of vessels appear (§ 29). Wolff did not find any internal organization in the vesicles constituting the organs of the plant, and he objected against the fantasy of microscopists who described non-existing structures. In the second remark (to 5 38) he wrote:
The extremely unreal substances , which Malpighi , (Nehemiah) Grew, and (Anthony van) Leeuwenhoek mentioned, are the fruits of their rich imaginations. Thus, for example, they assumed that those vesicles which I have described and interpreted are similar to the glands of animals; vessels must enter them, which because of their small size cannot be seen by any microscope. These glands discharge their juice in the vessels, which similarly occurs in the glands of animals . Why do we imagine that which has no trace in nature? Why, timidly, do we search everywhere for a miracle? Maybe for the reason that the clever production and wisdom of the Creator is revealed. It must not be forgotten that the quality of the machine can be judged, not by the number of its constituent parts, but by the excellence and the simplicity of its result.
The description of the emergence of the internal organization of plants is considered an argument in favor of epigenesis and against preformation. That same idea stands out in all statements on the development of plants, including consideration of the final formation of the leaves, formation of stem, roots, flowers, fruits, and seeds. For example, the leaves, which still do not possess their final form, finally become formed, and in their place new young non-formed leaflets originate. They are formed from the structureless substance of the vegetational bud; that is why in the place of the substance used for the formation of leaves, new structureless substance appears (5 53) .
In the second part of the dissertation there are factual data and considerations concerning animal development. The object of Wolff's investigation was the development of the chick embryo in the first stages of incubation. As in plants and in animals, the cause of development, according to Wolff, is the essential power and the ability of solidification of the living substance.
Wolff noticed in the 28-hour embryo a characteristic external form and position of the yolk. Concerning the internal structure, he found at this stage only the presence of globules scarcely connected with each other; the embryo was transparent and of semifluid consistency. It was impossible to differentiate either heart, vessels or red blood in it ($ 166). In the commentary to this paragraph Wolff dwelled on the fundamental argument of preformationists, who explained the invisibility of the preexisting structures by the insufficiency of their optical means, and also by the characteristics of the embryo itself, beginning with its transparency. First of all he took notice of the accusation he anticipated from his scientific opponents, and admitted that it is impossible to consider non-existing that which is beyond direct perception. That is why he also had mentioned that this principle has a character of sophism rather than truth. Since the particles from which all early rudiments of animal organs originate are globules visible by average magnification of the microscope, then it is impossible to say that the parts of the body of the embryo, if they are preformed, are beyond perception. "Thus," Wolff wrote, "the confirmation that the parts of the body of the embryo are latent due to their infinitely small size, and that they only become visible gradually later on is considered a fable." Continuing, he noticed that it is accurate to explain the way nature produces the parts of the organism, particularly in the formation of the extremities (5 217 and following), kidney ($ 220 and following), and so on. The transparency, which truly disturbs observation, he did not consider insurmountable. The body looks transparent in this case if it lies on another body, in it or under it. If the transparent formation is placed so that it will not be connected with other parts, then it cannot escape observation.
Undoubtedly, Wolff's claim that even "with the help of stronger lenses no one discovered those parts which are not perceived by the weaker magnification," is controversial. These words must not be considered as an expression of a scornful attitude on Wolff's part to the microscope as an instrument of scientific investigation. 4 Wolff apparentlymeant only that magnification by the microscope does not always find in the embryo the preformed organism, because the latter is not always present. Thus, Wolff declared in the German THEORIE that "the seminal animals (spermatozoa) — these are not the production of philosophy, but the result of Leeuwenhoek's polished glass" (p. 73). Here the sharp polemics are turned, of course, not to Leeuwenhoek's technical achievement in improving the microscope, but to the idea of animalculism, which supposes that in the spermatozoid, an animal exists preformed.
Wolff returned again to the question of whether it is possible to confirm the existence of unperceptible substances. His point of view is completely definite: the investigation must take into consideration only that which can be perceived and must judge the existence of substances by their detectable manifestations. Wolff's idea is presented in the following form:
I can, for example, very easily prove that in my bag there is no "fridrikhsdor"^ and no Dorida is present now in my room. You easily see that these determined substances are connected with determined manifestations, which in conformity with their nature cannot remain secret. The "fridrikhsdor" in my bag must be seen or be felt, and if Dorida is now present, then other signs of her presence must be available.
Comparing the embryo after 28 and 36 hours of incubation, Wolff discovered in the latter the beginning of the heart, in the form of a tube. This tube has the form of a third of a ring, still not pulsating and not connected with the arteries and veins. He, with surprise, noticed that during the short period between 28 and 36 hours essential changes in embryonic development take place C$ 167).
4. Thus, apparently, S. L. Sobol was ready to consider in his main work, ISTORIYA MIKROSKOPA I MIKROSKOPICHESKIKH ISSLEDOVANII V ROSSII V XVIII VEKE (History of the microscope and microscopical investigations in Russia in the 18th century) , Moscow: Akademii Nauk, 1949.
5. A golden coin.
The increase of size and the progressive formation shows that the embryo feeds on the egg substance, From here it follows, according to Wolff, that the nutritional particles pass from the egg to the embryo. And all of this is accomplished by means of a power. As this power is not the contraction of the heart and arteries or the muscles, since all these parts do not yet function or do not yet exist, and since the power acting here sends away the nutritional substances by a canal, then it, is consequently analogous to the essential power ($ 168) . Wolff considered that the nutritional juices in the embryo of the chicken move, as in plants, stimulated by the essential power, through the substance built from globules and deposited between these globules, and by this movement they increase the size of the embryo ($ 169) .
Animal substance, as plant substance, is characterized by the ability to solidify. This substance is soluble in water. While standing and during simultaneous slight heating, it gradually loses water and is transformed at the beginning into a more or less thick, and then hard body C$ 171) . Wolff considered that the ability of solidification of the animal substance is expressed in a lesser degree than that of the plant; its solidification occurs more slowly and it never reaches that degree which is characteristic for the plant substance. He confirmed this for the cellular tissue of animals, which never possesses that compactness and solidity possessed by the wood of even young trees (§ 172) .
Returning to his description of phenomena, Wolff turned attention to the moment which precedes the beginning of heart pulsation. In the umbilical vascular area (area vasculosa), the substance of the embryonic disc disintegrates into islets. Their number progressively increases, and their color changes from white to dark-yellow to red. It follows from that, that in other eggs practically in the same stage of development, where the fluid in the islets is slightly more red, the heart pulsates extremely animatedly ($ 179) . This description, stated nearly in Wolff's actual words, shows that he distinctly saw the formation and development of blood islets and the blood in them prior to the formation of the heart and to the beginning of its activity.
"Of the number of my most fortunate observations," Wolff wrote in a commentary to this paragraph, "I relate that which I am reprinting in Figures 7 and 8." I suppose that here it is possible to overhear nature when it is engaged in very important work, namely to see the transformation of fluid, stealing between the islets, into blood." The observation of the blood movement in the vascular area creates the impression that the vessels already existed (Fig. 3, 7). Examining this region under the microscope, Wolff did not see anything, however, except distintegrated heaps similar to those represented in Fig. 3, 4 aa. At this moment the true vessels are still not formed. Somewhat later on, head of the embryo and the first outlines of the brain, cerebellum and eyes are outlined, and "the rudiments of the vertebral column in the form of a chamber, filled with a homogenous substance are visible; but the rudiments of the extremities have not yet appeared" (Fig. 3, 10). At this point "the outlined cavities, filled with blood bodies are clearly visible" (§ 180) . The next step in the development of the vessels is the widening of their cavities at the expense of the increasing amount of blood and the increased thickening of the vascular walls; the globules making up the wall of the vessel appear more compressed. The most compact structure is possessed by the part of the wall near its opening; the farther from the axis of the vessel, the more the substances become softer and, finally, are transformed into the ordinary loose cellular tissue, filling the spaces between the vessels (§ 181) .
The processes of development of the chick embryos are distinctly connected with heat, obtained during incubation; if giving heat is discontinued, then all formative processes stop. Wolff considered that heat cannot be one of the specific causes of organic development, however, because "all physical changes in the world, to a certain extent ., are in need of heat, as to a resolving power. Without heat all will be turned into lifeless chaos" (§ 183) . Wolff saw the role of heat in embryonic development as leading to the dissolution of yolk, which then can be assimilated by the embryo (§ 186) . From these considerations this conclusion follows: "We do not know any other power, except heat and essential power, which shares in the formative processes... Therefore there is no basis for doubt that the supply of nutritional substances to the fetus., takes place under the influence of the essential power" ($ 187).
6. Here and later on, in the present chapter the references to figure are related to Table 11 of Wolff, reprinted in Figure 3 .
The subsequent statement was dedicated to the detailed description of how the vascular branches originate and the difference in formation of the veins and arteries; later Wolff addressed the development of vessels, nourishing of vessels, venous valves and anastomosis (§ 215 and 216). It is impossible not to agree with Wolff in these special observations, because in the discussion above, his method of embryological investigations and his fundamental opinions about developmental regularity have already become clear. In the conclusion, he compared development in plants and in animals; then he concluded that here and there the same formative principle acts. Any differences are expressed by the following: animals have a heart, which is absent in plants, and the latter have a central core and point of growth, which are absent in animals; but these differences do *not constitute the main point of plant and animal organisms The peculiarities of animals and plants are the expression of their form and structure only to the extent that these are machines. The essence of the animal and plant depends not so much upon the structure, as upon the activity of the essential power (§ 216) .
Then Wolff turned to the formation of organs, beginning from the development of the extremities. After 36 hours of incubation, even the rudiments of the extremities are absent (Fig. 3, 5). In this stage the outlines of the eyes, brain, cerebellum, medulla oblongata and spinal chord are visible in the vertebrae; only the spinal vertebrae are clear, while the lumbar vertebrae are slightly outlined. The periphery of the embryo is occupied by a "light substance," which Wolff called cellular and which he saw as composed of globules ($ 218). This substance gradually combines from both sides of the embryo in two definite points — on the level of the lumbar vertebrae and on the level of the heart. In these places prominences form (Fig. 3, 11), at the end of which the extremities develop. These prominences "truly are the first rudiments of the extremities" ($ 219) .
Figure 3. Table of drawings from K. F. Wolff's dissertation illustrating the development of the chicken.
1 . The embryonic spot (n) from the non-incubated egg in its natural size? b, c - circular position of the globules around the "center of conception" of the embryo (a) ; d- granules of yolk.
2. The embryonic spot after 28 hours of incubation in its natural size.
3. The same, under magnification? a - yolk membrane; b - embryo, included in saccule of water membrane.
4. The same, under somewhat less magnification; saccule (c) on the embryo extends and the last shines (e) ; a and b in correct rings and islets around the embryo; s - stretched membrane in fold.
5. The embryo at 36 hours; in the head "something of a beak," parts of brain and eyes with optic nerves; h - the spinal cord; d, e, f, g - vertebrae; c - substance surrounding the spinal cord; b- cavity of embryo membrane; k - heart.
6. Part of the vertebrae of the embryo represented in Figure 5, under greater magnification.
7. The embryonic spot in the ovum after 64 hours of incubation, in its natural size; v - yolk; d, f - islets: a - umbilical disc; b - rudiments of the vessels; n - mobile particle ("membrane" of Malpighi) ; e - embryo.
8. Region of embryonic spot of the previous embryo, in greater magnification; a - island of "white matter"; e, b - red fluid; d - boundary of the islet, as in Figure 7.
9. Part of the spinal column (a) and heart (c) of embryo of
64 hours; b - substance surrounding the vertebrae; d - cavity of the heart with blood globules .
10. Part of umbilical area after 72 hours of incubation; magnification larger than Figures 7 and 8; e, fc> - ducts, forming the vessels; m - boundary of the yolk.
11. Embryo after 96 hours of incubation; the vertebrae are visible (g) ; rudiments of extremities (r) and heart (c) .
12. Half of vertebra of embryo of 34 hours; c - substance surrounding the column; r - rudiment of the extremity.
13. Posterior part of the vertebra of the embryo of the fifth day; g - vertebra; r - extremity; b - condensation of substance, giving the origin of the rectum between extremities and the tail, and also kidney (d) ; e - remnants of urinary sac and membrane of the umbilical vessels .
14. Spirit preparation of chicken embryo, slightly magnified; - occiput; S - forehead; r - beak; v - vertebra; e - extremities; a - abdomen.
15 . Pelvis and lower part of the abdomen of the embryo represented in Figure 14; covers of the abdomen; b - red fluid, the same as under letter b in Figure 13; d - place of removed rectum; f - place where intestine passes in the still unformed substance e; c - substance giving the origin of the ureter.
16. Kidneys (b) and rectum (e) with caecum of embryo somewhat later than in Figure 14; c - ureters; d - the lower part of the rectum.
17. Nearly formed kidneys (b) ; c - ureters; d - rectum.
Wolff began his description of kidney development with demonstrations that up to the fourth and fifth days of incubation the embryos are deprived of any trace of the kidney. Only at this stage their first rudiments appear in the form of accumulated cellular tissue in the area of the lumbar vertebrae, joining the vertebral column from the front (Fig. 3, 13). From these rudiments, oval bodies of the kidneys are later formed. The ureters develop from their posterior end leading into the rectum (Figs. 3, 17, § 220 - 221) . Since Wolff made the first description of kidney development, the kidney in this stage of formation, or mesonephros, received the name Wolffian body. Wolff emphasized that the substance from which the organs form contains nothing except the slightly formed globules (§ 230) .
From the first two parts of Wolff's dissertation, which are almost without polemical discussions, it is clear that his task was to demonstrate with factual data how the development of plant and animal organisms is accomplished. Each of the examples he mentioned show satisfactorily that there is no pre-existence or preformation, and that the parts of the developed organism are formed anew from structureless material. To summarize: The first step is the formation of vesicles or globules which, through nutrition, increase in number and in this way form cellular tissues, the material for building organs. The generalization that these globules are the basis for both the rudiments of organs and the actual organs in both plants and animals holds paramount importance. It represents the basic doctrine about the composition of all the organic bodies from cells, i.e. the cell theory, which was formulated only eighty years later. Wolff's significance as one of the early contributors to the cellular theory has been repeatedly presented in literature' and need not be discussed here in further detail.
- See, for example, Z[akharie) S(aulovich) Katsnel'son, STO LET UCHENIYA KLETKE . ISTORIYA KLETOCHNOI TEORII (Hundred years study about the cell. History of the cellular theory) , Moscow: Akademii Nauk , 1939, and B. E. Raikov, pp. 54, 64 - 65. A. E. Gassinovich in the above-cited article about Wolff rightly warns against the categorical confirmation of Wolff's role in the substantiation of the cellular theory.
The third part of Wolff's German THEORIE , titled "On the organic bodies of nature and on their development generally," provides some concluding generalization. In order not to repeat what others had stated previously, Wolff considered it necessary to provide a summarized theory of development, briefly formulated in the form of definite propositions, and to confirm the correctness of the system with his own microscopical investigations. "Only at this time," Wolff wrote, "when all was completed and prepared for publication, did I read books which by chance proved to be identical to the conclusions in my dissertation" (§ 231). Of his predecessors who wrote about epigenesis, Wolff mentioned C. G. Ludwig and (John Turberville) Needham. All the others, according to Wolff, either agreed with Ludwig or defended clear error. Harvey, according to Wolff, adopted his opinion from Ludwig. He confirmed that the male semen affects the uterus in a way analogous to the way impressions arise in the brain of a painter who creates the likeness of the source of his impressions (§ 232) .
Wolff pointed out that the first principle of development, its moving power which he called the essential power, is a necessary condition of existence of animals and plants. Many authors and investigators had known this power for a long time, although they did not consider it the principle of development. Thus Ludwig, denying the necessity of this power for plants, suggested that the distribution of fluid in the plant organism was due to external causes. Later, Wolff noticed that the expansive and vegetative power, and also the power of resistance, which Needham had mentioned, were not identical with his own principle of development. In his work, "New observations on development, building and destruction of animal and plant substances" Needham called the expansive power of growing matter. Under the influence of this power, each separate point of matter attempts continuously to move away from its neighboring points, and Needham considered growth as extension. However, Wolff designated the essential power to be the ability to distribute and move the fluids somewhere else in the plant, i.e. something more determined than simple extension. Speaking about the extension of plant substance in vesicles and canals, he attributed the extension of substance not having the essential power to a mechanical pressure of penetrating fluid in this substance ($ 233). Wolff later noted that the necessary character of plant and animal substance (here he called this character the decisive cause) is its solidification ability, expressed in plants and animals in different degrees. His predecessors also mentioned this character, although they did not consider it as the principle of development. Thus, Needham spoke about the power of resistance, by means of which the particles of matter joined together. Wolff wrote: "In one unbearably tangled book, I could not find the author's reference to the power of resistance that sharply differs from the power of linkage, which is characteristic for all bodies of nature" (§ 234). Wolff hurled reproaches at authors who wrote about the development of an epigenetic spirit, who did not give any definite formative principle, and who "spoke about it as Galen had about digestion, when he confirmed that the last is conditioned by digestible power" (5 235) .
Contrary to his Latin dissertation, where Wolff criticized his epigenetic predecessors, in the German he considered in detail the opinions of opponents of the epigenesis theory, the supporters of preformation. Without touching upon the opinions of Haller, whom he respected, he directed his polemical discussion mainly at Bonnet, whose book was published two years before the publication of Wolff's German work. Wolff sharply attacked the theory of preformation, calling it a chimera, an idea taken from air. The nature of preformation he called pitiful and even simply rubbish. He decidedly rejected the idea; instead, in the German book, he defended the theory of epigenesis. The understanding of epigenesis, only casually mentioned in the Latin work, is widely used in the German. Characterizing epigenesis as formation of organic bodies, Wolff represented it as the real antithesis of preformation. While the latter hypothesis denies that the body undergoes formation (Ed. : since it is already preformed), epigenesis confirms this formation. Recognition that the hypothesis of predelineation is false at the same time confirms the truth of epigenesis (pp. 61 and 62). Epigenesis is real development in characteristics of all natural phenomena, including inorganic phenomena The rainbow appears suddenly, but it does not pre-exist in latent form. Epigenesis, as true development, appears in the organized bodies of nature, as is clear in examples of plant development from seeds and animals from ova.
Great interest is represented by Wolff's perspicacious discussion about the philosophical source of the predelineation hypothesis. He considered pre-established harmony to be Leibnitz's idea. Censure of the idea of preformation is, at the same time therefore, censure of Leibnitz's idealistic proposition. It is important to be aware of this, because the literature demonstrates the succession of the idea from Leibnitz to K. F. Wolff, and through philosophy, physics and botany from Leibnitz to Christian Wolff. If the influence of Leibnitz on K. F. Wolff cannot be denied completely, then Wolff's doctrine of essential power is not free from traces of this influence. The independence of Wolff's philosophical opinions must therefore not be overstated or overevaluated.
From the early facts and considerations already mentioned Wolff concluded the general laws of development, the most important of which are formulated as follows: the appearance of parts of the body and its organization are different processes; first, the part is isolated, and then organized. Essential for understanding Wolff's outlook is his confirmation that "all parts of any organic body are formed by means of this or that natural process" ($ 240) . He consequently denied the role of any unknown principle in development. The second chapter of his third part, summing up the investigation of development, begins with a definition of this understanding. Development is considered to be extremely general : Wolff includes all changes, either appearance of new parts and their formation by the extension of the substance of fluids in particular, or simple growth and maintenance of existing structures by means of nutrition C$ 241) .
The different forms of development in plants and animals represent manifestations of fundamental principles — the essential power and ability of solidification of the nutritional juices. Wolff again stresses the primacy of this essential power. "The essential power," Wolff stated, "together with the ability of solidification of the nutritional juices, forms the acting principle of all development, either in plants, or in animals" ($ 242). All other conditions or principles of development Wolff called accessories; in some way they promote the course of developmental process. Thus: "Except for the essential power and the ability of the developing substance to solidify, no other determining principle shares in the process of development. It follows that the developing organic bodies are not machines, but consist of non-organic substance" (§ 253) . In the commentary to this paragraph he puts the question, why does development need this non-organic substance? Because it possesses definite characters or because it builds in a definite way? Wolff, without deviation, assumed the first alternative, and as a proof he referred to the growth of plants in which the arising parts are composed only of non-organized mass. The organization, the structure in this non-organized matter, is manifested later on (§ 253).
From these considerations appears the general relation of Wolff to the mechanical explanation of vital phenomena, either normal or pathological. The title of paragraph 255 directly asks the question: "What is thought on this basis about the mechanical medicine?" The answer is given as follows: "If mechanical medicine is labelled that which explains the vital processes of the human body...., then it is clear that mechanical medicine is an imagined system, i.e. that which does not correspond to any thing in nature."
Commenting on this conclusion, Wolff characterized the opinions of the supporters of mechanical medicine: "The little which they have established hardly penetrates through the surface of matter, and the great part of that which concerns animal nature is either completely hypothetical or simply unknown" (5 255, remark 1). Below (Remark 2) Wolff indicated that the mechanical interpretation of vital phenomena only gives that explanation deceptive clarity. In the conclusion to this paragraph, Wolff turned to the characteristics of his own view, warning against hurried conclusions, because "If you, favorable reader, want to guess my opinion, then you easily can make a mistake. Further you can approach my opinions, if you think about Stahl's opinion . . . according to which the functions of our body must be attributed to a non-material spirit" (5 255, Remark 4) .
This declaration of Wolff must be considered carefully, and it must not be hastily concluded that he was a supporter of Stahl's animistic vitalism. Wolff's idea is greater in that he declared himself as a decisive opponent of mechanical understanding of vital phenomena and objected against the mechanists' consideration of non-organic processes. In order to imagine clearly Wolff's opinions, we must pursue a thought from one of the concluding paragraphs: "The basis for formation of the plant bud and animal embryo is contained only in the available substance capable of development, which is supplied by the essential power." And later: "For the appearance of the organized body of nature nothing is needed except a substance capable of development, which by some means is produced by nature."
In the last (third) chapter of the dissertation, Wolff's remark that attracts attention is that the working out and proofs of his assumed principles of development (the essential power and ability of solidification) must be continued in the future. Wolff pursued this in the investigations and theoretical explanations in both his THE FORMATION OF THE INTESTINE (1766 - 1768) and "On the Special Essential Power" (1789) .
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