Fig. 199. Section through the embryo of Agelena labyrinthica.
The section is from an embryo of the same age as fig. 200 A, and is represented with the ventral plate upwards. In the ventral plate is seen a keel-like thickening, which gives rise to the main mass of the mesoblast.
yk. yolk divided into large polygonal cells, in several of which nuclei are shewn.
The nuclei of the yolk cells during the above stages multiply rapidly, and cells are formed in the yolk which join the blastoderm; there can however be no doubt that the main increase in the cells of the blastoderm has been due to the division of the original blastoderm cells.
In the next stage I have been able to observe there is, in the place of the previous thickened half of the blastoderm, a well-developed ventral plate with a procephalic lobe in front, a caudal lobe behind, and an intermediate region marked by about three transverse grooves, indicating a division into segments. This plate is throughout two or more rows of cells thick, and the cells which form it are divided into two distinct layers—a columnar superficial layer of epiblast cells, and a deeper layer of mesoblast cells ([fig. 203] A). In the latter layer there are several very large cells which are in the act of passing from the yolk into the blastoderm. The identification of the structures visible in the previous stage with those visible in the present stage is to a great extent a matter of guess-work, but it appears to me probable that the primitive cumulus is still present as a slight prominence visible in surface views on the caudal lobe, and that the other thickened patch persists as the procephalic lobe. However this may be, the significance of the primitive cumulus appears to be that it is the part of the blastoderm where two rows of cells become first established[182].
The whole region of the blastoderm other than the ventral plate is formed of a single row of flattened epiblast cells. The yolk retains its original constitution.
By this stage the epiblast and mesoblast are distinctly differentiated, and the homologue of the hypoblast is to be sought for in the yolk cells. The yolk cells are not however entirely hypoblastic, since they continue for the greater part of the development to give rise to fresh cells which join the mesoblast.
The Spider’s blastoderm now resembles that of an Insect (except for the amnion) after the establishment of the mesoblast, and the mode of origin of the mesoblast in both groups is very similar, in that the longitudinal ridge-like thickening of the mesoblast shewn in [fig. 199] is probably the homologue of the mesoblastic groove of the Insects’ blastoderm.
The ventral plate continues to grow rapidly, and at a somewhat later stage ([fig. 200] B) there are six segments interposed between the procephalic and caudal lobes. The two anterior of these (ch and pd), especially the foremost, are less distinct than the remainder; and it is probable that both of them, and in any case the anterior one, are formed later than the three segments following. These two segments are the segments of the cheliceræ and pedipalpi. The four segments following belong to the four pairs of ambulatory legs. The segments form raised transverse bands separated by transverse grooves. There is at this stage a faintly marked groove extending along the median line of the ventral plate. This groove is mainly caused by the originally single mesoblastic plate having become divided throughout the whole region of the ventral plate, except possibly the procephalic lobes, into two bands, one on each side of the middle line ([fig. 203] B).