The Early Development of Julus Terrestris1

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Abstract
With regard to the segmentation I have described, it will be seen that it differs considerably from that seen by Metschnikoff (9), who describes it as total; the ovum being divided into two, four, &c, segments. I saw nothing of such a division, nor does Newport, who observed the eggs of the same species as I did, record any such appearances. Newport's observations were made on the eggs of a species found in Madeira; that is in a hot climate; and as regards segmentation were not carried on by means of sections. As the amount and distribution of the food-yolk has a great influence on the segmentation, I think it probable that in my species the segmentation differs slightly from that in the species investigated by Metschnikoff. The difference, however, consisting in the external segmentation of the ovum is not, I think, a very important one. The segmentation of Julus terrestris, as I have described it, shows a remarkable resemblance to that found in Amphipods by Ulianin (14). He describes an external segmentation by means of shallow furrows formed in the surface of the ovum, which is composed in great part of food-yolk; in each space thus marked out, a large amoeba-like mass of protoplasm provided with a nucleus is present; the division of these protoplasmic masses coincides with the formation of the furrows. When the blastoderm is just about to be formed the furrows disappear. At the close of segmentation, then the ovum is exactly like the ovum of Julus terrestris inasmuch as the segments are represented by protoplasmic masses each of which is provided with a nucleus. The formation of the blastoderm, as I have described it, agrees in the main with that given by Metschnikoff for Strongylosoma. According to this author, on the fifth day isolated masses of cells make their appearance on the surface of the ovum and spread themselves round it to form the blastoderm. He was unable to trace the origin, of these masses of cells. What he saw was precisely what I have described in the earlier part of this paper. The formation of the blastoderm in Julus is, then, such as is generally found in tracheate development. The cells which at the conclusion of the blastoderm formation in Julus remain within the yolk, represent the endoderm, and have apparently been overlooked by Metschnikoff. The mode of formation of the mesoderm almost exactly resembles that described by Balfour (16) for Spiders. According, however, to this author the greater part of the cells of the keel or ridge are derived from the ectoderm, whereas in Julus the ectoderm furnishes the greater part of them. Balfour states that the keel in Spiders is probably the homologue of the mesoblastic groove of the insect blastoderm. Patten (12) describes a median longitudinal furrow in the ventral plate of Phriganids which gives rise to the mesoblast and to part of the endoderm. In Peripatus (17) the mesoblast originates from the primitive streak, i. e. from the indifferent tissue behind the blastopore, which can be called neither ectoderm nor endoderm. I think that all these structures are homologous. With regard to the cells which, as I have already mentioned, are employed, neither in the formation of the keel nor at a later period in the formation of the mesenteron, but remain in the body cavity as mesoderm cells directly descended from endoderm--Balfour states that in Agelena, after the establishment of the hypoblast the cells remaining in the yolk are not entirely hypoblastic, since they continue for the greater part of the development to give rise to fresh cells, which join tlie mesoblast. This is exactly what happens in Julus. Metschnikoff has described the formation of the bands of mesoblast and their division into somites, but his figures are difficult to understand, as he has not drawn either the cell outlines or the nuclei. The formation of the ventral flexure has been described by Metschnikoff, and, as I have already mentioned, was first seen by Newport. The flexure is, as I have before said, formed between the sixth and seventh post-cephalic segments; that iss it marks off from the rest of the body the long eighth segment in which the tissues are very imperfectly differentiated, and from which the anal segment has yet to be cut off. It is from this imperfectly differentiated segment that the future additional body segments are formed in the later stages of development. The mesenteron of the adult animal is, as was pointed out to me by the late Professor Balfour, marked with a series of constrictions corresponding with the external segmentation of the body, but no trace of such constrictions has as yet appeared. The wide separation of the nerve-cords in the embryo has, so far as I know, not been pointed out by any author. I propose to reserve for a future paper a more full description of the development of the nervous system, the circulatory system, and the segmentation of the embryo, as well as the account of the appendages and other points connected with the further development of the embryo. The above investigations were entirely carried on in the Cambridge Morphological Laboratory.

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