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Cited by 2 publications
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Summary When skeleton‐forming cells of a donor sea urchin embryo are transplanted into a host embryo of another species, whose endoderm and mesoderm have previously been removed so that it only possesses ectoderm, a larva may be produced which is a chimaera consisting only of skin and skeleton. The donor skeleton is harmoniously situated in the host larva, thanks to the influence of the latter. But the skeleton affects the host inasmuch as it forces the latter to form larval processes. The skeletal structure is of the donor type. When skeleton‐forming cells are implanted into the whole larva of another species, an intermediate type of skeleton arises, with the exception of the skeletal rods, which occur only in the host form. These are developed exactly as in the host form. When skeleton‐forming cells are implanted into an embryo whose own skeleton‐forming cells have previously been removed, a skeleton develops which at first has the donor structure. Later on the host also supplies skeleton‐forming cells, and the skeleton which has already been formed gradually changes towards that of the host form. Hybrids obtained by cross‐fertilization of the same forms as those which made the chimaeras also have intermediate skeletons. When a species‐hybrid is made (A ♀×B ♂), its skeleton‐forming cells contain only maternal cytoplasm (A), but half maternal (A) and half paternal (B) chromatin. When the skeleton‐forming cells of such a hybrid are implanted into an embryo of the maternal species (A), whose skeleton‐forming cells contain both A cytoplasm and A chromatin alone, a hybrid chimaera is obtained, the skeleton‐forming cells of which contain cytoplasm of the maternal species (A) alone but chromatin of both species in the ratio of 3 A: I B. The skeletons are intermediate, but approach nearer to the maternal type. If the maternal component is weakened by the excision of some skeleton‐forming cells from the host before the implantation, then the skeleton is more definitely intermediate. The formation of a skeletal rod depends on two factors, the presence of the arm‐ectoderrn and of the corresponding skeleton‐forming cells. If the arm‐ectoderm is absent, the corresponding skeletal rod cannot be formed. If the arm‐ectoderm is present, and the skeleton‐forming cells are hybrids between a species which normally possesses a skeletal rod and one which lacks it, then the rod is not formed. Thus the absence of the skeleton‐forming factor is dominant to its presence. A study of normal skeleton formation gives the impression that the skeletogenous cytoplasm of Echinocyamus has a lower viscosity than that of Psammechinus. Protoplasmic viscosity seems to be one of the factors determining the particular structure of the skeleton. The fact that in sea‐urchin larvae with simple skeletons there appear “directed variations” tending towards the type of the more complicated forms is explicable in this manner. It was found to be possible, through the effects of high temperature and chemical substances, to influence larvae of Echinus very consi...
Summary When skeleton‐forming cells of a donor sea urchin embryo are transplanted into a host embryo of another species, whose endoderm and mesoderm have previously been removed so that it only possesses ectoderm, a larva may be produced which is a chimaera consisting only of skin and skeleton. The donor skeleton is harmoniously situated in the host larva, thanks to the influence of the latter. But the skeleton affects the host inasmuch as it forces the latter to form larval processes. The skeletal structure is of the donor type. When skeleton‐forming cells are implanted into the whole larva of another species, an intermediate type of skeleton arises, with the exception of the skeletal rods, which occur only in the host form. These are developed exactly as in the host form. When skeleton‐forming cells are implanted into an embryo whose own skeleton‐forming cells have previously been removed, a skeleton develops which at first has the donor structure. Later on the host also supplies skeleton‐forming cells, and the skeleton which has already been formed gradually changes towards that of the host form. Hybrids obtained by cross‐fertilization of the same forms as those which made the chimaeras also have intermediate skeletons. When a species‐hybrid is made (A ♀×B ♂), its skeleton‐forming cells contain only maternal cytoplasm (A), but half maternal (A) and half paternal (B) chromatin. When the skeleton‐forming cells of such a hybrid are implanted into an embryo of the maternal species (A), whose skeleton‐forming cells contain both A cytoplasm and A chromatin alone, a hybrid chimaera is obtained, the skeleton‐forming cells of which contain cytoplasm of the maternal species (A) alone but chromatin of both species in the ratio of 3 A: I B. The skeletons are intermediate, but approach nearer to the maternal type. If the maternal component is weakened by the excision of some skeleton‐forming cells from the host before the implantation, then the skeleton is more definitely intermediate. The formation of a skeletal rod depends on two factors, the presence of the arm‐ectoderrn and of the corresponding skeleton‐forming cells. If the arm‐ectoderm is absent, the corresponding skeletal rod cannot be formed. If the arm‐ectoderm is present, and the skeleton‐forming cells are hybrids between a species which normally possesses a skeletal rod and one which lacks it, then the rod is not formed. Thus the absence of the skeleton‐forming factor is dominant to its presence. A study of normal skeleton formation gives the impression that the skeletogenous cytoplasm of Echinocyamus has a lower viscosity than that of Psammechinus. Protoplasmic viscosity seems to be one of the factors determining the particular structure of the skeleton. The fact that in sea‐urchin larvae with simple skeletons there appear “directed variations” tending towards the type of the more complicated forms is explicable in this manner. It was found to be possible, through the effects of high temperature and chemical substances, to influence larvae of Echinus very consi...
Summary In the cleavage stages the sea urchin's egg can be divided into five transverse layers, an1, an2, veg1, veg2, and the micromeres (Fig. I). The ectoderm of the pluteus larva is derived from an1+ an2+ veg1, while veg2 gives rise to the secondary mesenchyme, the coelom, and the endoderm. The micromere material migrates into the blastocoele before gastrulation, forming the primary mesenchyme, which produces the skeletal rods. The position uf the skeleton‐forming cells and of the rods is determined by the ectoderm. The factors determining the cleavage type of the 16‐cell stage (8 meso‐, 4 macro‐, and 4 micromeres) seem to be (I) progressive changes in the cytoplasm, causing spindles formed a certain time after fertilization to lie in a certain direction, (2) the presence in the vegetative part of the egg of a region of micromere‐forming material, and (3) the activation of that material a certain time after fertilization. This leads to partial cleavage of isolated blastomeres, and to whole, intermediate or partial cleavage of fragments of undivided eggs, depending upon the time and plane of isolation. Whole eggs may also show partial cleavage. Differentiation is independent of the type of cleavage which the egg or fragment has undergone. The polarity (animal‐vegetative axis) of the egg is fairly stable, since it is not altered by centrifuging or by moderate stretching, and it is more or less retained in fragments. On the other hand, the polarity can be changed both by a greater degree of stretching and by placing animal and vegetative material in atypical relationship to one another. A new axis may then be induced, and the whole polarity may sometimes be reversed. A reversal can be brought about both by vegetative and by animal material. The dorso‐ventral axis is less stable, as it adjusts itself in accordance with the direction of stretching (perpendicular to the egg axis), not only to a considerable, but also to a moderate degree of stretching. After considerable stretching or constriction, both of which involve partial physiological isolation, as well as on complete isolation, the dorso‐ventral axis is spontaneously reversed in the dorsal half. In right and left halves the dorso‐ventral axis is maintained, and the larvae are more or less defective on thecut side. In starfish larvae a similar reversal of the right‐left axis may occur in right halves. After isolation animal material will form a considerably enlarged apical tuft and later develop only into ciliated cylinder epithelium. In veg., weg., and the micro‐meres, we find the faculty of checking the enlargement of the apical tuft and of causing the formation of stomodaeum, ciliated band, and pavement epithelium out of the animal material. Moreover, veg2 has the faculty of forming endoderm and skeletal cells, and under certain conditions also ectoderm. An endodermization of presumptive ectoderm can also be brought about by veg2, but this power of induction is much stronger in the micromeres. To explain the conditions in the sea urchin's egg we assume an a...
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