The works of Francis Maitland Balfour, Volume 1 (of 4)

by Francis M. Balfour

A section of the ovum at this stage is represented in Pl. 31, fig. 11. It is not quite certain in what direction the section is taken, but I think it probable it is somewhat oblique to the long axis. However this may be, the section shews that the whitish hemisphere of the blastoderm is formed of columnar cells, for the most part two or so layers deep, but that there is, not very far from the middle line, a wedge-shaped internal thickening of the blastoderm where the cells are several rows deep. With what part visible in surface view this thickened portion corresponds is not clear. To my mind it most probably corresponds to the larger white patch, in which case I have not got a section through the terminal prominence. In the other sections of the same embryo the wedge-shaped thickening was not so marked, but it, nevertheless, extended through all the sections. It appears to me probable that it constitutes a longitudinal thickened ridge of the blastoderm. In any case, it is clear that the white hemisphere of the blastoderm is a thickened portion of the blastoderm, and that the thickening is in part due to the cells being more columnar, and, in part, to their being more than one row deep, though they have not become divided into two distinct germinal layers. It is further clear that the increase in the number of cells in the thickened part of the blastoderm is, in the main, a result of the multiplication of the original single row of cells, while a careful examination of my sections proves that it is also partly due to cells, derived from the yolk, having been added to the blastoderm.

In the following stage which I have obtained (which cannot be very much older than the previous stage, because my specimens of it come from the same batch of eggs), a distinct and fairly circumscribed thickening forming the ventral surface of the embryo has become established. Though its component parts are somewhat indistinct, it appears to consist of a procephalic lobe, a less prominent caudal lobe, and an intermediate portion divided into about three segments; but its constituents cannot be clearly identified with the structures visible in the previous stage. I am inclined, however, to identify the anterior thickened area of the previous stage with the procephalic lobe, and a slight protuberance of the caudal portion (visible from the surface) with the primitive cumulus. I have, however, failed to meet with any trace of the cumulus in my sections.

To this stage, which forms the first of the second period of the larval history, I shall return, but it is necessary now to go back to the observations of Claparède and Balbiani.

There can, in the first place, be but little doubt that what I have called the primitive cumulus in my description is the structure so named by Claparède and Balbiani.

It is clear that Balbiani and Claparède have both failed to appreciate the importance of the organ, which my observations shew to be the part of the ventral thickening of the blastoderm where two rows of cells are first established, and therefore the point where the first traces of the future mesoblast becomes visible.

Though Claparède and Balbiani differ somewhat as to the position of the organ, they both make it last longer than I do: I feel certainly inclined to doubt whether Claparède is right in considering a body he figures after six segments are present, to be the same as the dorsal organ of the embryo before the formation of any segments, especially as all the stages between the two appear to have escaped him. In Agelena there is undoubtedly no organ in the position he gives when six segments are found.

Balbiani's observations accord fairly with my own up to the stage represented in fig. 2. Beyond this stage my own observations are not satisfactory, but I must state that I feel doubtful whether Balbiani is correct in his description of the gradual separation of the procephalic lobe and the cumulus, and the passage of the latter to the dorsal surface, and think it possible that he may have made a mistake as to which side of the procephalic lobe, in relation to the parts of the embryo, the cumulus is placed.

Although there appear to be grounds for doubting whether either Balbiani and Claparède are correct in the position they assign to the cumulus, my observations scarcely warrant me in being very definite in my statements on this head, but, as already mentioned, I am inclined to place the organ near the posterior end (and therefore, as will be afterwards shewn, in a somewhat dorsal situation) of the ventral embryonic thickening.

In my earliest stage of the third period there is present, as has already been stated, a procephalic lobe, and an indistinct and not very prominent caudal portion, and about three segments between the two. The definition of the parts of the blastoderm at this stage is still very imperfect, but from subsequent stages it appears to me probable that the first of the three segments is that of the first pair of ambulatory limbs, and that the segments of the cheliceræ and pedipalpi are formed later than those of the first three ambulatory appendages.

Balbiani believes that the segment of the cheliceræ is formed later than that of the six succeeding segments. He further concludes, from the fact that this segment is cut off from the procephalic portion in front, that it is really part of the procephalic lobe. I cannot accept the validity of this argument; though I am glad to find myself in, at any rate, partial harmony with the distinguished French embryologist as to the facts. Balbiani denies for this stage the existence of a caudal lobe. There is certainly, as is very well shewn in my longitudinal sections, a thickening of the blastoderm in the caudal region, though it is not so prominent in surface views as the procephalic lobe.