which cause it to divide into numerous cells, and cause these cells to arrange themselves upon the model of the parents which formed the egg and spermatozoa, and to imitate the peculiarities of the cells in each locality, making an eye where the parent had an eye, a gland where the parent had a gland-only the imitation is imperfect, the offspring is not absolutely the same as the parent. Evidently the fusion of the genoblast is the source of an increased vitality and of a formative power which is specific in each case, i. e., the action and result of which is predetermined.

. This marvelous formative power has always excited the interest and astonishment of naturalists. It is one of the fundamental distinctions of life, since no similar power occurs in inorganic

It is important to note, therefore, that it must enter into. all cells, otherwise some of them would not form in the right place and manner. That other cells than the fertilized ovum contain such a power is shown by the formation of buds and strobila, and more strikingly by the development of pseudova. In the latter instance, the development begins with a cell arising in the ovary, and which resembles an ordinary egg very closely. Such cells are formed in various animals, notably in the plant lice, but, although they are so like eggs, the pseudova differ by being capable of developing into a complete animal without impregnation.

For want of space, it is impossible to describe the formation of buds and strobila, let it therefore suffice to say, that the reproduction depends in both cases upon the separation of a cluster of cells (instead of a single cell or pseudovum) from the body of the parent. This cluster grows up into a complete animal, in which the structure of the parent, or sometimes of the grandparent, is imitated by the action of the formative force of the cluster of cells. Hence it is evident that a similar power is bestowed upon several cells, which is the thesis we started to prove.

III-SEGMENTATION AND THE FORMATION OF THE

GASTRULA.

After the impregnation has been completed, and the two pronuclei have fused, to form the segmentation-nucleus, there usually follows a period of quiescence, during which no visible changes occur. It is not known whether such a period is always intercalated in the course of development; but it has been observed frequently. After this pause the process of segmentation begins, which has for

its essential purpose the multiplication of cells; the further history of the egg is a description of the way in which the cells, constantly on the increase, arrange themselves in definite order, until they have gradually created, or, more truly, become, the adult animal. The object of embryology is to discover the laws according to which this arrangement is developed.

We, of necessity, begin with a study of the process of segmentation; but the details are so numerous that we can indicate only a few of them. The first result is the formation of two sets of cells. In one set the cells are small; in the other set they are large. Except in the sponges, the small cells form the outside covering of the body, appearing as a sac, or vesicle. The large cells form the lining of the digestive canal, or primitive stomach, and are, therefore, enclosed in the outer vesicle made by the small cells. It appears that this disposition arises in two entirely distinct ways. First, the cells formed by segmentation arrange themselves in the shape of a sphere, hollow inside, and its walls consisting of a continuous layer of cells. One half is composed of small cells; the other half of large cells. Second, the result of segmentation is likewise a hollow sphere, but with double walls; the outer wall of small cells, the inner wall of large cells. In both cases the sphere transforms itself into a so-called gastrula. In the first instance, the large cells become inverted inwards, or, in technical language, invaginated; while the small cells grow down and around the others, until they encase them, leaving only a small opening, the primitive mouth. In the second instance, an opening breaks through both walls, thus making a mouth. This

cells is slight, but evident. In B, the difference is more marked,

and fairly represents a gastrula of Amphioxus. In C, the difference is very great, and corresponds to a form observed in certain Gasteropods. In D, the inner set is no longer separated into distinct cells, although there are a number of nuclei, each of which marks the center of a future cell. In such an instance we should regard the whole inner set as a nutritive yolk, not yet transformed into a definite cell-layer. This figure is particularly instructive, because it shows that what we call the yolk is not something distinct from the germ, but really belongs to the inner layer of the embryo. E shows a similar egg, in which the outer set of cells has not yet grown around the yolk. This outer layer was called by the earlier embryologists the blastoderm, in all those eggs with a great deal of yolk. F shows the same egg not in section, but seen from the outer surface, to exhibit the cap of small cells, or the blastoderm, resting upon the large yolk. Those eggs in which the differFIG. 13.-Formation of ence in size between the two sets of cells is the blastoderm in Oniscus

not excessive (A-C) are called holoblastic, murarius, after Bobretzky. while those in which the yolk remains more or less intact for a considerable time (D-F) are termed meroblastic.

In order more fully to illustrate the peculiarities of the process of segmentation, it is necessary to consider the holoblastic eggs further. Fig. 13 represents an actual section of an egg of the sow-bug, Oniscus, after Bobretzky, corresponding very nearly to the diagram E, of Fig. 12. Fig. 14 is a similar section through the egg of a moth (Pieris crataegi), and shows a number of nuclei, each surrounded by a little mass of protoplasm, and scattered irregularly through the yolk. Their number gradually increases, and each one becomes the center of a distinct cell. This is merely a peculiar modification of the ordinary method of cell division into two equal parts, for in the moths and butterflies and some other animals the large yolk divides gradually, by forming several nuclei, and so breaking up into a considerable number of cells piled up one over the other. We shall have oc

FIG. 14.

Sec

casion to recur to this matter in speaking of the development of vertebrates.

The embryology of sponges is important because they do not have any gastrula. It will be described in our next article. Except in the sponges, the small cells form the outside layer and are called the ectoderm, while the large cells form the inside layer, or entoderm. In England the attempt has been made to substitute epiblast for ectoderm, and hypoblast for entoderm, but the change seems to me useless and confusing. In face of the present tendency to substitute new and difficult for old and simple names every protest is desirable. Compounding English polysyllables from Latin and Greek confers, in most cases, no benefit to science. The coining of such terms ought to be restricted in its application to things which have no accepted name and for which no straightforward English term can be found.

The next progress after the formation of the ectoderm and ento

derm does not occur among all animals, but only in those above the Colenterates. I refer to the development of a distinct middle layer of cells, the mesoderm, situated as shown in Fig. 15, between the two primitive

layers. Of the origin

and characteristics of the mesoderm I shall treat in the next article.

A great many embryos live in the water, and have they have any muscles.

the power of locomotion long before For this purpose the ectoderm in these forms is provided with cilia or vibratile hairs, which may be longer (Fig. 15) or shorter. In most free embryos, moving by cilia, we find distinct bands, along which the cilia are more developed and powerful; as the ciliated bands are often pigmented, while the rest of the embryo is transparent or light-colored, they are very conspicuous. shall have to recur to them.

We

The authorities for the general views advanced above are the

discussions in a long series of special papers. Prof. Haeckel1 has written a great deal upon the gastrula and its significance, and has published several popular works on embryology. Unfortunately, he is inaccurate and untrustworthy to a degree surpassing any other scientific writer I can recall, for on almost every page are mistakes it requires little knowledge to detect. He is, therefore, utterly useless to the beginner. I mention this, not alone as my personal conviction, but also as the judgment of competent and distinguished critics, some of whom are even more severe in their condemnation. For these considerations I shall not quote Hacckel as an authority. The references to some of the special papers I have consulted will be given hereafter.

E. GENERAL PAPERS ON THE GERM LAYERS.

31. Agassiz, Alexander. Critique de la Gastræa theorie. (Traduit par Schneider.) Arch. Zool. expt. Tome IV, p. IX (1875). Also Mem. Amer. Acad. x, No. 3. 32. Lankester, E. Ray. On the Primitive Cell Layers of the Embryo, etc., etc. Ann. and Mag. of Nat. History, Vol. XI (1873), p. 321-338.

33

Notes on the Embryology and Classification of the Animal Kingdom, etc. Quart. Journ. Micros. Sci. 1877. p. 399.

34. Minot, C. S. Recent Investigations of Embryologists, etc. Proc. Boston S. N. H. Vol. XIX, p. 165. (A brief summary.)

35. Moquin-Tandon. De quelques applications de l'Embryologie à la classification méthodique des animaux. Ann. Sci. Nat. Zool., 11 (1875), Art. 7.

36. Salensky. Bemerkungen über Haeckel's Gastræatheorie. Archiv f. Naturges. Bd. 1. Jahrg. 40 (1874).

37. Semper, Carl. Kritische Gänge. No. III. Die Keimblätter theorie. Verh. phys. med. Gesell. Würzburg. Bd. (1873), p. 202.

1 "Professor Haeckel's principal articles on the Gastrula are to be found in the Jena Zeitschrift für Naturwissenschaften, Band. vIII, p. 1, and Biologische Studien, 2tes Heft. 1877. Haeckel introduced the term Gastrula, and his writings and speculations have afforded a powerful stimulus to embryological research.”