Book - Comparative Embryology of the Vertebrates 3-8
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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.
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Part III The Development of Primitive Embryonic Form
Part III - The Development of Primitive Embryonic Form: 6. Cleavage (Segmentation) and Blastulation | 7. The Chordate Blastula and Its Significance | 8. The Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning | 9. Gastrulation | 10. Tubulation and Extension of the Major Organ-forming Areas: Development of Primitive Body Form | 11. Basic Features of Vertebrate Morphogenesis
The Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning
A. Introduction
In the preceding two chapters the blastula is defined as a morphological entity composed of six, presumptive, organ-forming areas — areas which are poised and ready for the next phase of development or gastrulation. However, the attainment of this morphological condition with its presumptive, organ-forming areas is valid and fruitful in a developmental way only if it has developed within certain physiological conditions which serve as a spark to initiate gastrulation and carry it through to its completion.
The physiological conditions of the blastula are attained, as are its morphological characteristics, through a process of differentiation. Moreover, during the development of the blastula, different areas acquire different abilities to undergo physiological change and, hence, possess different abilities or powers of differentiation. To state the matter differently, the various, presumptive, organ-forming areas of the blastula have acquired different abilities not only in their power to produce specific organs of the future body of the embryo, but also in that some presumptive areas possess this propensity in a greater degree than do other areas. However, at this point, certain terms in common usage relating to the problem of differentiation are defined in order that a better understanding may be obtained concerning the ability to differentiate on the part of the presumptive, organ-forming areas of the late blastula.
B. Problem of Differentiation
1. Definition of Differentiation; Kinds of Differentiation
The word differentiation is applied to that phase of development when a cell, a group of cells, cell product experiences a change which results in a persistent alteration of its activities. Under ordinary conditions an alteration in structure or function is the only visible evidence that such a change has occurred.
To illustrate these matters, let us recall the conditions involved in the maturation of the egg. A subtle change occurs within the primitive oogonium which causes it to enlarge and to grow. This growth results in an increase in size and change in structure of both the cytoplasm and the nucleus. A little later, as the egg approaches that condition which is called maturity, observable morphological changes of the nucleus occur which accompany or initiate an invisible change in behavior. These latter changes make the egg fertilizable. Here we have illustrated, first of all, a subtle, invisible, biochemical change in the oogonium which arouses the formation of visible morphological changes in the oocyte and, secondly, a morphological change (i.e., nuclear maturation) which accompanies an invisible physiological transformation.
Another illustration will prove profitable. Let us recall the development of the mammary-gland tissue (fig. 58). Through the action of the lactogenic (luteotrophic) hormone, LTH, the cells of the various acini of the fully developed gland begin to secrete milk. The acini, it will be recalled, were caused to differentiate as a result of the presence of progesterone. Similarly, the various parts of the complicated duct system were stimulated to differentiate from a very rudimentary condition by the presence of estrogenic hormone. Earlier in development, however, the particular area of the body from which the duct rudiments ultimately arose was conditioned by a change which dictated the origin of the duct rudiments from the cells of this area and restricted their origin from other areas.
In the foregoing history of the mammary gland, various types of differentiation are exemplified. The final elaboration of milk from the acinous cells is effected by a change in the activity of the cells under the influence of LTH. The type of change which brings about the functional activities of a structure is called physiological differentiation. The morphological changes in the cells which result in the formation of the duct system and the acini are examples of morphological differentiation. On the other hand, the invisible, subtle change or changes which originally altered the respective cells of the nipple area and, thereby, ordained or determined that the cells in this particular locale should produce duct and nipple tissue is an example of biochemical differentiation or chemodifferentiation. Chemodifferentiation, morphological differentiation, and physiological differentiation, therefore, represent the three types or levels of differentiation. Moreover, all of these differentiations stem from a persistent change in the fundamental activities of cells or cell parts.
It should be observed further that chemodifferentiation represents the initial step in the entire differentiation process, for it is this change which determines or restricts the future possible activities and changes which the cell or cells in a particular area may experience. Also, in many cases, differentiation appears to arise as a result of stimuli which are applied to the cell or cells externally. That is, internal changes within a cell may be called forth by an environmental change applied to the cell from without.
In embryological thinking, therefore, the word differentiation implies a process of becoming something new and different from an antecedent, lessdifferentiated condition. But beyond this, differentiation also connotes a certain suitableness or purposefulness of the structure which is differentiated. Such a connotation, however, applies only to normal embryonic differentiation; abnormal growths and monstrosities of many kinds may fulfill the first phase (i.e., of producing something new) of differentiation as defined in the first sentence of this paragraph, but they do not satisfy the criteria of purpose and of suitableness within the organized economy of the developing body as a whole. It is important to keep the latter implications in mind, for various structures may appear to be vestigial or aberrant during embryonic development, nevertheless their presence may assume an important, purposeful status in the ultimate scheme which constructs the organization of the developing body.
2. Self-differentiation and Dependent Differentiation
In the amphibian, very late blastula and beginning gastrula, the presumptive, chordamesodermal area, when undisturbed and in its normal position in the embryo, eventually differentiates into notochordal and mesodermal tissues. This is true also when it is transplanted to other positions. That is, at this period in the history of the chordamesodermal cells the ability resides within the cells to differentiate into notochordal and mesodermal structures. Consequently, these cells are not dependent upon surrounding or external factors to induce or call forth differentiation in these specific directions. Embryonic cells in this condition are described as self-differentiating (Roux). Similarly, the entodermal area with its potential subareas of liver, foregut, and intestine develops by itself and this area does not rely upon stimuli from other contiguous cells to realize a specific potency. On the other hand, the presumptive, neural plate region at this time is dependent upon the inducing influence of the chordamesodermal cells during the process of gastrulation for its future realization as neural tissue. This area has little inherent ability to differentiate neural tissue and is described, therefore, as being in a state of dependent differentiation (Roux). Furthermore, the presumptive skin ectoderm (i.e., epidermis), if left alone, will proceed to epidermize during gastrulation, but foreign influences, such as transplantation, into the future neural plate area may induce neural plate cells to form from the presumptive skin ectoderm (fig. 183). The differentiation of neural cells from any of the ectodermal cells of the late blastula thus is dependent upon special influencing factors applied to the cells from without.
C. Concept of Potency in Relation to Differentiation
1. Definition of Potency
The word potency, as used in the field of embryology, refers to that property of a cell which enables it to undergo differentiation. From this viewpoint, potency may be defined as the power or ability of a cell to give origin to a specific kind of cell or structure or to various kinds of cells and structures.
It is questionable, in a fundamental sense, whether potency actually is gained or lost during development. It may be that the expression of a given kind of potency, resulting in the formation of a specific type of cell, is merely the result of a restriction imposed upon other potentialities by certain modifying factors, while the total or latent potency remains relatively constant. All types of differentiated cells, from this point of view, basically are totipotent; that is, they possess the latent power to give origin to all the kinds of cells and tissues of the particular animal species to which they belong.
The specific potencies which denote the normal development of particular organs undoubtedly have their respective, although often quite devious, connections with the fertilized egg. However, one must concede the origin of abnormal or acquired potency values due to the insinuation of special inductive or modifying factors which disturb the expression of normal potency value. For example, tumors and other abnormal growths and tissue distortions may be examples of such special potencies induced by special conditions which upset the mechanism controlling normal potency expression.
2. Some Terms Used to Describe Different States of Potency
a. Totipotency and Harmonious Totipotency
The word totipotent, as applied to embryonic development, was introduced into embryological theory by Wilhelm Roux, and it refers to the power or ability of an early blastomere or blastomeres of a particular animal species to give origin to the many different types of cells and structures characteristic of the individual species. Speculation concerning the meaning of totipotency of a single blastomere received encouragement from the discovery by Hans Driesch, in 1891, that an isolated blastomere of the tWo- or four-cell stage of the cleaving, sea-urchin's egg could give origin to a "perfect larva." Driesch described this condition as constituting an equipotential state, while Roux referred to it as a totipotential condition. As the word totipotential seems more fitting and better suited to describe the condition than the word equipotential, which simply means equal potency, the word lotipotency is used herein. The word omnipotent is sometimes used to describe the totipotent condition; as it has connotations of supreme power, it will not be used.
The totipotent state is a concept which may be considered in different ways. In many instances it has been used as described above, namely, as a potency condition that has within it the ability to produce a perfect embryo or individual. The word also has been used, however, to describe a condition which is capable of giving origin to all or nearly all the cells and tissues of the body in a haphazard way but which are not necessarily organized to produce a normally formed body of the particular species. Therefore, as a basis for clear thinking, it is well to define two kinds of totipotency, namely, totipotency and harmonious totipotency. The former term is used to describe the ability of a cell or cell group to give origin to all or nearly all the different cells and tissues of the particular species to which it belongs, but it is lacking in the ability to organize them into an harmonious organism. Harmonious totipotency, on the other hand, is used to denote a condition which has the above ability to produce the various types of tissues of the species, but possesses, in addition, the power to develop a perfectly organized body.
The fertilized egg or the naturally parthenogenetic egg constitutes an harmonious totipotential system. This condition is true also of isolated blastomeres of the two- or iour-blastomere stage of the sea-urchin development, as mentioned above, of the two-cell state of Amphioxus, or of the first twoblastomere stage of the frog's egg when the first cleavage plane bisects the gray crescent. However, in the eight-cell stage in these forms, potency becomes more limited in the respective cells of the embryo. Restriction of potency, therefore, is indicated by a restriction of power to develop into a variety of cells and tissues, and potency restriction is a characteristic of cleavage and the blastulative process (figs. 61; 163 A; 163B). When a stage is reached in which the cells of a particular area are limited in potency value to the expression of one type of cell or tissue, the condition is spoken of as one of unipotency. A pluripotent state, on the other hand, is a condition in which the potency is not so limited, and two or more types of tissues may be derived from the cell or cells.
b. Determination and Potency Limitation
The limitation or restriction of potency, therefore, may form a part of the process of differentiation; as such, it is a characteristic feature of embryonic development. Potency limitation, however, is not always the result of the differentiation process. For instance, in the development of the oocyte in the ovary, the building up of the various conditions, characteristic of the totipotent state, is a feature of the differentiation of the oocyte.
The word determination is applied to those unknown and invisible changes occurring within a cell or cells which effect a limitation or restriction of potency. As a result of this potency limitation, differentiation becomes restricted to a specific channel of development, denoting a particular kind of cell or structure. Ultimately, by the activities of limiting influences upon the resulting blastomeres during cleavage, the totipotent condition of the mature egg becomes dismembered and segregated into a patchwork or mosaic of general areas of the blastula, each area having a generalized, presumptive, organ-forming potency. As we have already observed, in the mature chordate blastula there are six of these major, presumptive organ-forming areas (five if we regard the two mesodermal areas as one). By the application of other limiting influences during gastrulation or the next phase of development, each of these general areas becomes divided into minor areas which are limited to a potency value of a particular organ or part of an organ. The process which brings about the determination of individual organs or parts of organs is called individuation.
When potency limitation has reduced generalized and greater potency value to the status of a general organ system (e.g., nervous system or digestive system) with the determination (i.e., individuation) of particular organs within such a system, the condition is described as one of rigid or irrevocable determination. Such tissues, transplanted to other parts of the embryo favorable for their development, tend to remain limited to an expression of one inherent potency value and do not give origin to different kinds of tissues or organs. Thus, determined liver rudiment will differentiate into liver tissue, stomach rudiment into stomach tissue, forebrain material into forebrain tissue, etc.
In many instances determination within a group of cells is brought about because of their position in the developing organism and not because of intrinsic, self-differentiating conditions within the cells. Because their position foreordains their determination in the future, the condition is spoken of as positional or presumptive determination. For example, in the late amphibian blastula, the composite ectodermal area of the epiblast will become divided, during the next phase of development, into epidermal and neural areas as a result of the influences at work during gastrulation, especially the activities of the chordamesodermal area. Therefore, one may regard these areas as already determined, in a presumptive sense, even in the late blastula, although their actual determination as definite epidermal and neural tissue will not occur until later.
As stated in the preceding paragraphs, determination is the result of potency limitation or inhibition. However, there is another aspect to determination, namely, potency expression, which simply means potency release or development. Potency expression, probably, is due to an activating stimulus (Spemann, '38). Consequently, the individuation of a particular organ structure within a larger system of organs is the result of two synchronous processes:
- inhibition of potency or potencies and
- release or calling forth of a specific kind of potency (Wiggles worth, '48).
Associated with the phenomenon of potency inhibition or limitation is the loss of power for regulation. Consequently, individuation and the loss of regulative power appear to proceed synchronously in any group of cells.
c. Prospective Potency and Prospective Fate
Prospective fate is the end or destiny that a group of cells normally reaches in its differentiation during its normal course of development in the embryo. The presumptive epidermal area of the late blastula differentiates normally into skin epidermis. This is its prospective fate. Its prospective potency, however, is greater, for under certain circumstances it may be induced, by transplantation to other areas of the late blastula, to form other tissue, e.g., neural plate cells or mesodermal tissues.
d. Autonomous Potency
Autonomous potency is the inherent ability which a group of cells possesses to differentiate into a definite structure or structures, e.g., notochord, stomach, or liver rudiments of the late blastula of the frog.
Versatility of autonomous potency is the inherent ability which a group of cells possesses to differentiate, when isolated under cultural conditions outside the embryo, into tissues not normally developed from the particular cell group in normal development. In the amphibian late blastula this is true of the notochordal and somitic areas of the chordamesodermal area, which may give origin to skin or neural plate tissue under these artificially imposed conditions.
e. Competence
Certain areas of the late amphibian blastula have the ability to differentiate into diverse structures under the stimulus of varied influence. Consequently, we say that these areas have competence for the production of this or that structure. The word competence is used to denote all of the possible reactions which a group of cells may produce under various sorts of stimulations. The entodermal area of the late amphibian blastula and early gastrula has great power for self-differentiation but no competence, whereas the general, neural plate-epidermal area has competence but little power of self differentiation (see p. 375). On the other hand, the notochord, mesodermal area possesses both competence and the ability for self-differentiation.
Competence appears to be a function of a developmental time sequence. That is, the time or period of development is all important, for a particular area may possess competence only at a single, optimum period of development. The word competence is sometimes used to supersede the other terms of potency or potentiality (Needham, '42, p. 112).
D. The Blastula in Relation to Twinning
1. Some Definitions
a. Dizygotic or Fraternal Twins
Fraternal twins arise from the fertilization of two separate eggs in a species which normally produces one egg in the reproductive cycle, as, for example, in the human species. Essentially, fraternal twins are much the same as the “siblings†of a human family (i.e., the members born as a result of separate pregnancies) or the members of a litter of several young produced during a single pregnancy in animals, such as cats, dogs, pigs, etc. Fraternal twins are often called “false twins.â€
b. Monozygotic or Identical Twins
This condition is known as "true twinning" and it results from the development of two embryos from a single egg. Such twins presumably have an identical genetic composition.
c. Polyembryony
Polyembrony is a condition in which several embryos normally arise from one egg. It occurs regularly in armadillos (Dasypopidae) where one ovum gives origin normally to four identical embryos (fig. 186).
2. Basis of True or Identical Twinning
The work of Driesch (1891) on the cleaving, sea-urchin egg and that of Wilson (1893) on the isolated blastomeres of Amphioxus mentioned above initiated the approach to a scientific understanding of monozygotic or identical twinning. Numerous studies have been made in the intervening years on the developing eggs of various animal species, vertebrate and invertebrate, and from these studies has emerged the present concept concerning the matter of twinning. True twinning appears to arise from four, requisite, fundamental, morphological and physiological conditions. These conditions are as follows:
- there must be a sufficient protoplasmic substrate;
- the substrate must contain all the organ-forming stuffs necessary to assure totipotency, that is, to produce all the necessary organs;
- an organization center or the ability to develop such a center must be present in order that the various organs may be integrated into an harmonious whole; and
- the ability or faculty for regulation, that is, the power to rearrange materials as well as to reproduce and compensate for the loss of substance, must be present.
3. Some Experimentally Produced, Twinning Conditions
The isolation of the first two blastomeres in the sea-urchin egg and in Amphioxus with the production of complete embryos from each blastomere constriction isolates the organizer areas in the dorsal portion of the early gastrula. (B) Later development of the dorsal portion isolated in (A). (C) Later development of ventral portion of gastrula isolated in (A). (D) Constriction of organizer area of early gastrula into two halves. (E) Result of constriction made in (D). Constrictions were made at 2-cell stage.
Fig. 183. Early gastrula of darkly pigmented Triton taeniatus with a small piece of presumptive ectoderm of T. cristatus lightly pigmented inserted into the presumptive, neural plate area shown in (A). (B) Later stage of development. (C) Cross section of the later embryo. I hc lighter eye region shown to the right was derived from the original implant from T. cristatus, (After Spemann, '38.)
Fig. 184. Demonstration that the presence of the organizer region or organization center is necessary for development. (Redrawn from Spemann, '38.) (A) Hair-loop
Fig. 185. Twinning in teleost fishes. (After Morgan, '34; Embryology and Genetics, Columbia University Press, pp. 102-104. A, B, C from Rauber; D from Stockard.) In certain teleost fishes, especially in the trout, under certain environmental conditions, two or more organization centers arise in the early gastrula. (A-C) These represent such conditions. If they lie opposite each other as in (A), the resulting embryos often appear as in (D). If they lie nearer each other as in (B) or (C), a two-headed monster may be produced.
has been described in Chapter 6, In these cases all the conditions mentioned above are fulfilled. However, in the case of the isolation of the first two blastomeres in Stye la described in Chapter 6, evidently conditions (1), (2), and (3) are present in each blastomere when the two blastomeres are separated, but (4) is absent and only half embryos result. That is, each blastomere has been determined as either a right or left blastomere; with this determination of potency, the power for regulation is lost. In the frog, if the first two blastomeres are separated when the first cleavage plane bisects the gray crescent, all four conditions are present and two tadpoles result. If, however, the first cleavage plane separates the gray-crescent material mainly into one blastomere while the other gets little or none, the blastomere containing the gray-crescent material will be able to satisfy all the requirements above, and it, consequently, develops a normal embryo. However, the other blastomere lacks (2), (3), and (4) and, as a result, forms a mere mass of cells. Again, animal pole blastomeres, even when they contain the gray-crescent material, when separated entirely from the yolk blastomeres, fail to go beyond the late blastular or beginning gastrular state (Vintemberger, '36). Such animal pole blastomeres appear to lack requirements (I), (2), and possibly (3) above. Many other illustrations of embryological experiments could be given, establishing the necessity for the presence of all the above conditions. Successful whole embryos have resulted in the amphibia when the two-cell stage and beginning gastrula is bisected in such a manner that each half contains half of the chordamesodermal field and yolk substance; that is, each will contain half of the organization center (fig. 184).
Fig. 186. Polyembryony or the development of multiple embryos in the armadillo, Tatusia novemcincta. (After Patterson, '13.) (A) Separate centers of organization in the early blastocyst. (B) Later stage in development of multiple embryos. Each embryo is connected with a common amniotic vesicle. (C) Section through organization centers a and b in (A). The two centers of organization are indicated by thickenings at right and left. (D) Later development of four embryos, the normal procedure from one fertilized egg in this species.
Monozygotic twinning occurs occasionally under normal conditions in the teleost fishes. In these cases, separate centers of organization arise in the blastoderm, as shown in figure 185. When they arise on opposite sides of the blastoderm, as shown in figure 185 A, twins arise which may later become fused ventrally (fig. 185D). When the centers of organization arise as shown in figure I85B, C, the embryos become fused laterally. Stockard ('21) found that by arresting development in the trout or in the blastoderm of Fundutus for a period of time during the late blastula, either by exposure to low temperatures or a lack of oxygen, twinning conditions were produced. The arrest of development probably allows separate centers of organfzation to arise. Normally, one center of organization makes its appearance in the late blastula of these fishes, becomes dominant, and thus suppresses the tendency toward totipotency in other parts of the blastoderm. However, in the cases of arrested development, a physiological isolation of different areas of the blastoderm evidently occurs, and two organization centers arise which forthwith proceed to organize separate embryos in the single blastoderm. Conditions appear more favorable for twinning in the trout blastoderm than in Fundulus. After the late blastular period is past and gastrulation begins, i.e., after one organization center definitely has been established, Stockard found that twinning could not be produced.
In the Texas armadillo, Tatusia novemcincta, Patterson ('13) found that, in the relatively late blastocyst (blastula), two centers of organization arise, and that, a little later, each of these buds into two separate organization centers, producing four organization centers in the blastula (fig. 186A-C). Each of these centers organizes a separate embryo; hence, under normal conditions, four embryos (polyembryony) are developed from each fertilized egg (fig. 186D).
It is interesting in connection with the experiments mentioned by Stockard above, that the blastocyst (blastula) in Tatusia normally lies free in the uterus for about three weeks before becoming implanted upon the uterus. It may be that this free period of blastocystic existence results in a slowing down of development, permitting the origin of separate organization centers. In harmony with this concept, Patterson ('13) failed to find mitotic conditions in the blastoderms of the blastocysts during this period.
In the chick it is possible to produce twinning conditions by separating the anterior end (Hensen's node) of the early primitive streak into two parts along the median axis of the developing embryo. Twins fused at the caudal end may be produced under these conditions. In the duck egg, Wolff and Lutz ('47) found that if the early blastoderm is cut through the primitive node area (fig. 187A), two embryos are produced as in figure 187 A'. However, if the primitive node and primitive streak are split antero-posteriorly, as indicated in figure 1876, two embryos, placed as in figure 187B', are produced.
It is evident, therefore, that in the production of monozygotic twins, condition (3) or the presence of the ability to produce an organization center is of greatest importance. In the case of the separation of the two blastomeres of the two-cell stage in Amphioxus or of the division of the dorsal lip of the early gastrula of the amphibian by a hair loop, as shown in figure 184, a mechanical division and separation of the ability to produce an organization center in each blastomere (Amphioxus) or of the separation into two centers of the organization center already produced (Amphibia) is achieved. Once these centers are isolated, they act independently, producing twin conditions, providing the substrate is competent. Similar conditions evidently are produced in the duck-embryo experiments of Wolff and Lutz referred to above.
In some teleost blastulae, e.g., Fundidus and Salmo, during the earlier period of development, it has been found possible to separate the early blastoderm into various groups of cells (Oppenheimer, '47) or into quadrants (Luther, '36), .and a condition of totipotency is established in each part. Totipotency appears thus to be a generalized characteristic in certain teleost blastoderms during the earlier phases of blastular development. Harmonious totipotency, however, appears not to be achieved in any one part of the blastodisc of these species during the early conditions of blastular formation. During the development of the late blastula, however, the posterior quadrant normally acquires a dominant condition together with a faculty for producing harmonious totipotency. The other totipotent areas then become suppressed. These basic conditions, therefore, serve to explain the experiments by Stockard ('21) referred to above, where two organization centers tend to become dominant as a result of isolating physiological conditions which tend to interfere with the processes working toward the development of but one center of organization. This probable explanation of the twinning conditions in the teleost blastoderm suggests strongly that the separation and isolation of separate organization centers is a fundamental condition necessary for the production of monozygotic or true twinning.
Fig. 187. Isolation of the organization center in the early duck embryo. (From Dalcq, '49, after Wolff and Lutz.) (A') Derived from blastoderm cut as in (A). (B') Derived from blastoderm cut as in (B).
It becomes apparent, therefore, that, in the development of the trout blastoderm (blastula), the development oj an area which possesses a dominant organization center is an important aspect of blastulation. In other blastulae, the seat or area of the organization center apparently is established at an earlier period, as, for example, the gray crescent in the amphibian egg which appears to be associated with the organization center during the late blastula state. Similarly, in the teleost fish, Carassius, totipotency appears to be limited to one part of the early blastula (Tung and Tung, '43).
It also follows from the analysis in the foregoing paragraphs that in the production of polyembryony in the armadillo or of spontaneous twinning in forms, such as the trout (Salmo), a generalized totipotency throughout the early blastoderm is a prerequisite condition. When a single dominant area once assumes totipotency, it tends to suppress and control the surrounding areas, probably because it succeeds in “monopolizing†certain, substrate, “food†substances (Dalcq, '49).
E. Importance of the Organization Center of the Late Blastula
It is also evident that one of the main functions of cleavage and blastulation is the formation of a physiological, or organization, center which must be present to dominate and direct the course of development during the next stage of development. Consequently, the elaboration of a blastocoel with the various, presumptive, organ-forming areas properly oriented in relation to it is not enough. A definite physiological condition entrenched within the so-called organization center must be present to arouse and direct the movement of the major, organ-forming areas during gastrulation.
Bibliography
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Wigglesworth, V. B. 1948. The role of the cell in determination. Symposia of the Soc. for Exper. Biol. No. II. Academic Press, Inc., New York.
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