Imágenes de páginas

is the process of tissue differentiation which emphasizes the contractile power in the muscle cell, at the same time limiting and finally eliminating the morphogenic power, and which gives the sex cells morphogenic power in such marked degree while it deprives them largely of contractile power. In a former paper, I stated this view thus: “The segmentation nucleus of metazoa contains, as in the infusoria, both micro and macro nuclear elements, but these are retained in varying proportions in its descendants, i.e., in the cells of the adult organism. Through a process of division of labor the power of rejuvenescence becomes restricted to comparatively few of the cells derived from the segmentation nucleus.”

While Minot's views are in part borne out by the conditions in Cymatogaster, the italicised part of the quotation below finds no support, and is negatived by all the observations made in Cymatogaster. His conclusion, as translated by me, is: “Somatic cells are simply cells in which the activity of heredity is prevented by senescence, viz. : tissue differentiation, but the somatic cells can, under favorable conditions, be translated into the rejuvenated stage and then develop the most complete or, at least, more complete, hereditary power."



The sex cells originally segregated retain their individuality, but undergo a measureable change between the time of their segregation and 7 min. long larvæ. Soon after the larva has reached a length of 7 mm., the sex cells begin to divide. In the meanwhile they have migrated laterad and lie, for the most part, in a longitudinal groove formed by a duplication of the peritoneum into which a few peritoneal cells have also migrated. In one such case an extra sex ridge was formed much further forward than usual, in connection with a few sex cells which were accidentally belated in their migration. The peritoneal cells which have migrated into the sex ridge give rise to the entire stroma of the future sex glands, and together with the sex cells form a core quite distinct from the covering

6 Bull. U. S. Fish Comm., XII, 442, 1894.

Size of larva.

of peritoneum. Posteriorly the sex ridges of the two sides are united into a single ridge. There is considerable variation in the rate of segmentation in larvæ of the same size, but the following table will give an idea of the segmentation and the number of cells in successive stages :

No. of sex cells. No. of generations from fertilization. 45–5 mm. 9-15



6-9 12 39–143

7-9 15-17 638-2280

11-13 sexes distinct. 16-25 2200-8000

13-15 The sexes can first be distinguished not by the differences in the sex cells, but in the tunic of peritoneal cells. A small groove on the outer ventral part of the sex ridge is the first indication of the ovarian cavity and the surest criterion of the female. In the male the sex gland remains much more circular in cross section and no groove is developed. Much later histological differences in the sex cells themselves can be made out. The long slender chromatin threads of the female cell just before dividing are represented in the male by short, thick bars.




(Continued from paye 18.3.) Lamarck and Defrance earnestly engaged in study of fossil shells, and the former, in 1802, reconstructed the system of conchology and introduced into it the new species collected by the latter from the strata underlying the city of Paris and quarried for the construction of its buildings. Six years previous to this Cuvier had established the different specific character of fossil and living elephants and he devoted himself to researches throughout the remainder of his life. Jameson, in 1808, pointed out the nature of all the rocks and the mode in which they were formed, and made use of the observations of Desmorest, who, in 1768, traced the origin of basalt to the crater of volcanoes.

In 1807 the Geological Society of London was established with the professed object of encouraging the collection of data and the making of observations. In 1819 the Society published a map of England by the aid of Greenough. About the same time Buch prepared a similar map of a large part of Germany. A geological survey of France was ordered in 1822 by the French government, and as a result a geological map was published in 1841. Conybeare and Phillips published a treatise on the “Geology of England and Wales,” in 1821. In 1814 Aiken published his work on mineralogy, which had a large circulation at home and in this country. Previous to this Sowerby published a work on “British Mineralogy, illustrated with colored plates,” but the date of which I do not know. The publication of the Geological Map of England, in 1815, by Smith, may be said to form an epoch in the history of geology.

In 1809 Maclure published an article on “ Observations on the Geology of the U. S., explanatory of a Geological Map," and he is rightly called the father of American geology. He visited all parts of the Union and all the principleming ni districts of Europe. In 1817 he presented a report to the “Phil. osophical Society of Philadelphia” of his work, and accompanied it with a colored map. In 1816 and 1817 he visited the Antilles and published a paper on their geology. In 1810 Bruce, of New York, published the first purely scientific journal supported by original American contributions. His journal was devoted principally to mineralogy and geology. Science was also promoted by the collections in the colleges and societies, and by those made by scientific men. In 1816 Cleveland published a treatise on mineralogy. In 1818 Dana published a detailed report on the mineralogy and geology of Boston and vicinity. In the same year the American Journal of Science was first published. The first geological survey made by State authority was that of North Carolina in 1824.

In 1830 the Principles of Geology, by Lyell, appeared and has most powerfully influenced the direction of scientific thought in the 19th century. It broke down the belief in the necessity of stupendous convulsions in past times. He adopted and improved the views of Hutton, eliminating the baseless theories mingled with them. He rendered great service in elucidating North American geology, and published his travels on this continent in 1845 and 1849. His“ Geological Evidences of the Antiquity of Man," published in 1863, startled the public by its advocacy of Darwin's theory in the “ Origin of Species."

And so the science has advanced with rapid strides and is solving the problems that are constantly arising in regard to our planet, and upon its fixed data are based many of the fundamental principles of philosophy.

Having considered the history of the progress of geology, let us now consider its aim and the fundamental principles upon which the geologist bases his work.

In the broadest sense, geology is the science whose province is the planet upon which we live, its history from the beginning to the present, including changes which have occurred in regard to the condition at different periods, its several physiographic features, its atmosphere, temperatures, and aqueous bodies, and its life at different stages. In a nutshell, the evolutionary progress of the earth.

The narrow or commonly accepted view does not consider the changes that have occurred, other than those that occurred to the visible portion of the earth. Back of what is supposed to be the earliest formation, it does not attempt to go.

The latter view is sufficient for the ordinary geologist or for the geologist who does not care to speculate on hypotheses which refer to the origin of the earth ; but to the geologist who is anxious to grapple with problems which require a drawing upon the imagination for solution, this is not enough. Chemists are not satisfied to study a drop of water, but they are anxious to know its origin; its composition is not sufficient for them. Botanists and zoologists desire to know the origin of plants and animals, not merely their structural and physiological features.

Geologists who study the earth, not merely to satisfy their own curiosity as to the present condition of things, but for the purpose of advancing the science, and unraveling the mysteries of the past, in order to produce a history of the planet as accurately as human knowledge in its present condition will permit, are only satisfied with the broad and comprehensive view.

Geology, by the aid of astronomy and physics, therefore, begins with a great nebulous mass, of which all celestial bodies were component parts. It traces the evolution of each body, and that of the earth in particular. Starting when the earth was thrown off as a ring of cloudy or gaseous elements, it traces it through its transformation into a sphere of molten matter surrounded with gases, through which the parent body, the sun, could not penetrate. We learn of the war that existed between the congealing surface and the liquid interior in which the former came off victorious, and formed a crust through which the latter seldom broke. Then began the war between the condensing vapors and the heated crust, in which the latter succumbed to the overpowering element that fell upon it and fairly covered it.

Geology tells us of the life that existed in this mighty ocean after it became sufficiently cooled, and in the powerful internal movements that resulted in the upheaval of masses of rock that were to be the nuclei of the present continents, the history and the formation of which is traced with great minuteness, and the life of each is described with great care, from the lowest forms to the highest, and also the period in which each form lived.

There are several principles by which the geologist is guided in answering the questions that continually arise as he studies the earth with its many characteristics.

1 In the first place, he understands that geology is an inductive science. That is, it is a process of demonstration in which a general truth is gathered from an examination of a selfevident truth. Let me illustrate : From the study of modern glaciers he learns certain facts in regard to conditions necessary for their formation, their modes of action, and the results of those actions.

« AnteriorContinuar »